Welcome to GASR.
People request a variety of things, including drawings (OCs mostly), IMVU screenshot edits/repaints, and pixels (mostly for badges). A badge is something users create and share on IMVU that are pixelled and can vary in sizes from 20x20 and up.
File size and image dimensions are varied, but most people like a 160x220 version of anything they order to use as a display image on the forum and on IMVU.
If you haven't heard of IMVU before, it's a 3D chat website that the forum was created for, but GASR has slowly opened it's doors to everyone, from anywhere, offering any type of artwork. (http://www.imvu.com/)
About the avatar issue; Forumotion has a size limit for display pictures set to 64 KB. If your image is larger than that, it will not let you upload it and will give you that error message. A work-around to this is to upload your avatar image to a photo hosting site (such as photobucket, tinypic, imgur, etc.) and paste the direct link in this box: