There Is A Limit to Signature Size, Which is 500Pixels(Width) By 200Pixels(Height).
Now, when you find/create/request your image, go into you Profile (link located up the top of the page). After that click the link for "Signature".
Now instead of uploading the picture onto the forum (thus using up our data) you want to go to www.tinypic.com And upload it onto there. copy the url that has [img] [/imag] tags on either side, then paste it into the box on our website. You'll see what i mean when you do it all. If you have any issues tell me and i'll try to help ^_^
Thanks,
xACEx