To do a collage, go to a photo uploading site.. My favorite is www.photobucket.com
and upload the pics you want. There is a drop down or add-on menu which says "e-z collage" and there you can put as many pics in the sig as you want. However, as I said, this site automatically resizes the sigs so if you have a lot of fish the image will get smaller and smaller (depending on the amount of pics in it).
When you save the collage then just copy the [img] code an paste it in the "edit signature" in your user cp here :D