Hello.
I made this tutorial just by a Support post.
The question was,"How to resize external linked (not attached) images automatically in IPB posts."
Well the solution is very simple,
In you admin panel :
Go to Settings > System Settings
Then go to Forums Tab > Topics, Polls and Posts
Find : Automatic resize Image force size and set the pixels by just entering the no
Note :
1) The number you enter is only for with. Also enter only number like 500 and not 500px
2) The height will automatically be determined and scaled down.
3) This will affect all posts. So this is not custom. Remember before changing.
4) By default, value is zero. This will resize the width to fit in the forum post.
I hope you find this helpful.
Best of luck,
Again, just incase if you're doubting my idea, I tested it on my live working forum.
So you can believe me.