The Modeler allows importing images with GIF, JPEG or PNG format. In principle there is no limitation on the size of the image, even though the image library in the Modeler shows all images as small icons. When you use an image on a form, the image is shown using its actual size.
A nice website containing a free icon library is famfamfam. We use these icons a lot in our projects, and in the Modeler itself.
Another good icon website that I always use is icon archive.
It would be a cool feather to be abel to upload .svg ... or complete custom icons sets.