Please note that this section is for questions regarding the forum itself - it is not for fan game-related questions. If you have a question about a fan game, ask in the appropriate thread.
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Select "Edit Signature" from the drop down menu or "Your Signature" directly from your User CP
Take the URL of your image and place it within the
brackets in the available space provided. Take note that some sites already supply the IMG code for you, so all you'd have to do is copy and paste that into the available space for your signature.
Select Preview to make sure it's how you want it and it's within the limits, and if it is, press Save.
It's also suggested that if you're using an image, it's better to save it onto your computer and upload it to a site such as Photobucket, Tinypic, imgur, or any other site similar to these of your preference. Sites such as these also have the IMG code ready, so again, all you'd have to do is copy and paste it into the field for editing your signature.
That's really all there is to it. I hope this wasn't too hard to follow and that it helps you out.