Content Tool - Steam Workshop - Upload Preview error image error message persists

1. Please describe as detailed as possible what you did and which problem that leads to.

When I tried to upload image with size: 4,16 MB (4 367 871 bytes) and resolution 2560 x 1440 as a preview image using the content tool's Workshop Upload Preview Image feature, I get the following error:

This error returns when you close it and it keeps repeating until I kill the contentool.exe



EDIT: Same happens when the same action is succesful (I made image smaller (filesize):
https://i.imgur.com/BwdETM2.gifv

Sorry, not exactly sure what you meant with that question.

If you meant, how did I try to close that error message, then:
I tried close it by pressing OK, close it through X and even Alt-F4. None of them helped.

If you meant, how did I finally get the preview image uploaded:
When I reduced the size of the image (by converting it from .png to .jpg), the upload was successful, however as you see from the first post EDIT gif, that message also got stuck (had to once again kill the contentool). But the upload itself was succesful.

Or did you mean something else with the question?