I'm not a support guy from Upwork, but here are the sizes I use based on my observations:
1) Project featured image: 340x253px;
2) Images inside the project: 760x440px.
The images should be .jpg, .jpeg, .gif, or .png file format with a maximum size of 5 mb.
Seems like UpWork could very easily provide ratio info for uploading images right where you're uploading them. The fact that we have to have a conversation about this in a forum seems to be an obvious teller that the UI is not as friendly as people are used to with other sites.
Good day, I have been downloading my Upwork profile as well and I used pdf. Now I am using my profile and there is no issue about it.
Just keep trying, use pdf.