I have tried several times to upload an image and description to my multiplayer mod. Whenever I try it says "Submitting, Please wait..." but never completes the upload. I am using a png resized from 2560x1440 pixels (1.86MB) to 1080x608 pixels (1.19MB).
Can you reply with the image here? Maybe Development can upload it to your mod for you. What's the name of the mod?
It might be that you need to refresh the screen a few times (press f5 on the keyboard).