AIM Expressions?

I want to be able to upload my own AIM expressions (buddy icons at least) but every time I try to upload the images I want to use AIM tells me that it is too large. I downloaded these buddy icons from a website and they are the right size as far as being 48X48, but I can't imagine why some icons work (Sweeney Todd ones off the website work fine) and some don't (Flight of the Conchords ones off their website). Any suggestions on how to get AIM to sponsor a new FotC AIM Expression or how to get the icons from the website to work?