Personally, I use Gyazo cap or photobucket.
https://gyazo.com/
http://s1186.photobucket.com/
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Gyazo:
Simply download the capture program, double click it and drag the box over the picture you want to use as your avatar/signature.
Copy the URL when it opens in your web browser automatically and add .png onto the end of the URL.
Go to the forum and click the picture button BBCode and insert the URL with .png on the end and click Ok for the picture to display.
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Photobucket:
Save the picture to your computer.
Create an account on photobucket and click upload.
Upload the photo and copy either the direct link and paste it into the insert picture box, or copy the forums link which already has the [ img ]<Link>[ /img ] BBCode already attached and just paste it into the text box, when you hit save/post then the picture should then actually turn into an image.
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To be honest, if you're looking for a high end quality signature, then use photobucket as it will save and upload the picture in the best quality, but personally I think gyazo, or any other screen cap program does the job.
Hope I helped,
Matt