Yes and no... (don't you hate that?)

There is no size (dimension, not file size) limit for uploading files, however, since Flash (the technology used to build the main image viewer) cannot handle images that are more than 5000 pixels in either width or height, anything larger than 5000 pixels will be automatically resized down to just under 5000 pixles so that it can be easily viewed with optimal efficiency.

If you're presenting web designs then the optimal setting is to have them 100% at 72dpi so they appear as they would when built in HTML and CSS in a browser.