When attempting to upload an image bigger than the max size as a Profile Header or User Card on my profile, I get the following:
Sorry, that file is too big (maximum size is 4096kb). Why not upload your large file to a cloud sharing service, then paste the link?
“Pasting the link” isn’t possible for these images, so perhaps the message could be something different for these two image upload situations.
Edit: Actually, the image I tried to upload was 2.11 MB, which is actually smaller than 4096kb (4 MB), isn’t it? The dimensions were 4096 x 1536 pixels.