1. Upload it to free imagehoster like
Photobucket or
Imageshack2. Copy the link of the image that you've just upload (the one that direct, not the thumbnail one)
3. Opens your forum profile
4. Paste the link to signature box, put
after the link.
That's it, you're done!
EDIT : You can also copy the image URL/link (whatever you want to call it) in AM after you uploaded it. Open the admin page, locate your logo then right-click on the image, then click on copy image URL, or any option that involve copying image URL address (different on every browser), after that just do steps 3 and 4, then you're done.