I thinks that it depends where do you want to upload it. In the comments section if i want to upload a picture it says no more than 100.2 MB which its a lot for a picture.
What I do when I want to upload a picture is to bring it up on a separate window and then minimize the picture until it's about 1/8 the size of my whole computer screen. Then I click ctrl + 4 so I can take a screenshot of only that window. This makes the photo much smaller but when you upload it it'll be a normal size. Just a bit blurry on the finer details.