Thanks for reaching out to us and sorry to hear that you’ve run into this issue!
I’ve just tried uploading a PNG on Windows 10 from here, and it seems to work normally. Could you please email us an example PNG to support [at] ribbet.com and just mention this issue and the error message or further details about what happens when you do try to upload it (e.g. it crashes, it comes in blank etc). We will investigate further to see if we can get to the bottom of the problem.