This issue has 2 parts that may or may not be solved with the same solution.
On the Publish page, after uploading a model and getting it's screenshot, if you then change the file to an image, that image's resolution will be forced to the resolution/aspect ratio of the model screenshot that was displayed previously.
After you have an image in this forced resolution, if you go to upload another file, but then hit cancel, an error is displayed and the upload button disappears until the page is refreshed.
This issue has 2 parts that may or may not be solved with the same solution.
On the Publish page, after uploading a model and getting it's screenshot, if you then change the file to an image, that image's resolution will be forced to the resolution/aspect ratio of the model screenshot that was displayed previously.
After you have an image in this forced resolution, if you go to upload another file, but then hit cancel, an error is displayed and the upload button disappears until the page is refreshed.