i did it this way and it worked:
change code fromTemplates/core/front/global/userPhoto with this one:
{{if $member->member_id and \IPS\Member::loggedIn()->canAccessModule( \IPS\Application\Module::get( 'core', 'members' ) )}}
{{$memberURL = ( $warningRef ) ? $member->url()->setQueryString( 'wr', $warningRef ) : $member->url();}}
<a href="{$memberURL}" {{if $hovercard}}data-ipsHover data-ipsHover-target="{$memberURL->setQueryString( 'do', 'hovercard' )}"{{endif}} class="ipsUserPhoto ipsUserPhoto_{$size}{{if $classes}} {$classes}{{endif}}" title="{lang="view_user_profile" sprintf="$member->name"}">
<img src='{$member->photo}' alt='{$member->name}' itemprop="image">
</a>
{{else}}
<span class='ipsUserPhoto ipsUserPhoto_{$size} {{if $classes}}{$classes}{{endif}}'>
<img src='{$member->photo}' alt='{$member->name}' itemprop="image">
</span>
{{endif}}
Then go to Templates/forums/front/topics/postContainer and change the whole code with this one:
{{$idField = $comment::$databaseColumnId;}}
{{$itemClassSafe = str_replace( '\\', '_', mb_substr( $comment::$itemClass, 4 ) );}}
{{if $comment->isIgnored()}}
<div class='ipsComment ipsComment_ignored ipsType_light' id='elIgnoreComment_{$comment->$idField}' data-ignoreCommentID='elComment_{$comment->$idField}' data-ignoreUserID='{$comment->author()->member_id}'>
{lang="ignoring_content" sprintf="$comment->author()->name"} <a href='#elIgnoreComment_{$comment->$idField}_menu' data-ipsMenu data-ipsMenu-menuID='elIgnoreComment_{$comment->$idField}_menu' data-ipsMenu-appendTo='#elIgnoreComment_{$comment->$idField}' data-action="ignoreOptions" title='{lang="see_post_ignore_options"}' class='ipsType_blendLinks'>{lang="options"} <i class='fa fa-caret-down'></i></a>
<ul class='ipsMenu ipsHide' id='elIgnoreComment_{$comment->$idField}_menu'>
<li class='ipsMenu_item ipsJS_show' data-ipsMenuValue='showPost'><a href='#'>{lang="show_this_post"}</a></li>
<li class='ipsMenu_sep ipsJS_show'><hr></li>
<li class='ipsMenu_item' data-ipsMenuValue='stopIgnoring'><a href='{url="app=core&module=system&controller=ignore&do=remove&id={$comment->author()->member_id}" seoTemplate="ignore"}'>{lang="stop_ignoring_posts_by" sprintf="$comment->author()->name"}</a></li>
<li class='ipsMenu_item'><a href='{url="app=core&module=members&controller=ignore" seoTemplate="ignore"}'>{lang="change_ignore_preferences"}</a></li>
</ul>
</div>
{{endif}}
<a id='comment-{$comment->$idField}'></a>
<article itemscope {{if $item->isQuestion() && !$comment->new_topic}}itemprop="suggestedAnswer {{if $comment->post_bwoptions['best_answer']}}acceptedAnswer{{endif}}" itemtype="http://schema.org/Answer"{{else}}itemtype="http://schema.org/Comment"{{endif}} id='elComment_{$comment->$idField}' class='cPost ipsBox {{if $otherClasses}}{$otherClasses}{{endif}} ipsComment {{if settings.reputation_highlight and $comment->reputation() >= settings.reputation_highlight}}ipsComment_popular{{endif}} ipsComment_parent ipsClearfix ipsClear ipsColumns ipsColumns_noSpacing ipsColumns_collapsePhone {{if $comment->isIgnored()}}ipsHide{{endif}} {{if $comment->hidden()}}ipsModerated{{endif}}'>
{{if $item->isQuestion() and !$comment->new_topic}}
{template="postRating" group="topics" app="forums" params="$item, $comment, $votes"}
{{endif}}
<aside class='ipsComment_author cAuthorPane ipsColumn ipsColumn_medium'>
<h3 class='ipsType_sectionHead cAuthorPane_author ipsType_blendLinks ipsType_break' itemprop="creator" itemscope itemtype="http://schema.org/Person"><strong itemprop="name">{$comment->author()->link( $comment->warningRef() )|raw}</strong> <span class='ipsResponsive_showPhone ipsResponsive_inline'> {template="reputationBadge" group="global" app="core" params="$comment->author()"}</span></h3>
<ul class='cAuthorPane_info ipsList_reset'>
{{if $comment->author()->member_title && $comment->author()->member_id}}
<li class='ipsResponsive_hidePhone ipsType_break'>{$comment->author()->member_title}</li>
{{elseif $comment->author()->rank['title'] && $comment->author()->member_id}}
<li class='ipsResponsive_hidePhone ipsType_break'>{$comment->author()->rank['title']}</li>
{{endif}}
{{if $comment->author()->rank['image'] && $comment->author()->member_id}}
<li class='ipsResponsive_hidePhone'>{$comment->author()->rank['image']|raw}</li>
{{endif}}
<li class='cAuthorPane_photo'>
{{if $comment->author()->pp_main_photo}}
<a href='{url="app=core&module=members&controller=profile&id={$comment->author()->member_id}-{$comment->author()->name}" seoTemplate="profile" seoTitle="$seoName"}' data-ipsHover data-ipsHover-target='{url="app=core&module=members&controller=profile&id={$comment->author()->member_id}-{$comment->author()->name}&do=hovercard" seoTemplate="profile" seoTitle="$seoName"}' title="{lang="view_user_profile" sprintf="$name"}">
<span class='ipsUserPhoto ipsUserPhoto_variable'>
<img src="{setting="base_url"}uploads/{$comment->author()->pp_main_photo}" />
</span>
</a>
{{else}}
<a href='{url="app=core&module=members&controller=profile&id={$comment->author()->member_id}-{$comment->author()->name}" seoTemplate="profile" seoTitle="$seoName"}' data-ipsHover data-ipsHover-target='{url="app=core&module=members&controller=profile&id={$comment->author()->member_id}-{$comment->author()->name}&do=hovercard" seoTemplate="profile" seoTitle="$seoName"}' title="{lang="view_user_profile" sprintf="$name"}">
<span class='ipsUserPhoto ipsUserPhoto_variable'>
<img src="<Path_to>/default_photo.png" />
</span>
</a>
{{endif}}
</li>
<li>{expression="\IPS\Member\Group::load( $comment->author()->member_group_id )->formattedName" raw="true"}</li>
{{if \IPS\Member\Group::load( $comment->author()->member_group_id )->g_icon }}
<li class='ipsResponsive_hidePhone'><img src='{file="$comment->author()->group['g_icon']" extension="core_Theme"}' alt='' class='cAuthorGroupIcon'></li>
{{endif}}
{{if $comment->author()->member_id}}
<li class='ipsResponsive_hidePhone'>{template="reputationBadge" group="global" app="core" params="$comment->author()"}</li>
<li class='ipsType_light'>{lang="member_post_count" pluralize="$comment->author()->member_posts"}</li>
{{if $comment->author()->reputationImage()}}
<li class='ipsPad_half ipsResponsive_hidePhone'>
<img src='{file="$comment->author()->reputationImage()" extension="core_Theme"}' title='{{if $comment->author()->reputation()}}{$comment->author()->reputation()}{{endif}}' alt=''>
</li>
{{endif}}
{template="customFieldsDisplay" group="global" app="core" params="$comment->author()"}
{{endif}}
</ul>
</aside>
<div class='ipsColumn ipsColumn_fluid'>
{template="post" group="topics" app="forums" params="$item, $comment, $item::$formLangPrefix . 'comment', $item::$application, $item::$module, $itemClassSafe"}
</div>
</article>
Then go to CSS/core/front/custom/custom.css and add:
.cAuthorPane_photo img{
max-width: 150px;
max-height: 300px;
}
Then go to Admin CP/Groups and edit everygroup, tab Social, seti Maximum profile photo width/height to 300.