I tried many times to upload various images for my mod and it kept failing silently. I finally looked at the HTML for the form and discovered that the images must be 1920 x 1080, and once I sized my images accordingly it worked. This seems like a requirement that should be specified, and an error should be displayed on failure. I'm using a Chromium based browser (Brave Version 1.7.98 Chromium: 81.0.4044.113).