If I may and to underscore the points made by Mimi above, from the 'Upload Images' page ('The Fine Print'):
"Please upload the largest possible version of the image (but under 3200 pixels in width and height), we will create the smaller versions automatically."
And maximum file size is actually unrestricted.
Though, 10 MB seems to be the cutoff ... once you factor in internet connectivity. Over that, and most likely your upload will 'time out'.