@Martin Fogarty#516 The Landscapepainter replied on this one. Here's the thread:
https://www.twoinchbrush.com/forum/d/53-posting-new-paintings-here
When uploading paintings, at the very top of the page you will find a blue colored button called 'Choose image'. Click on that and you can upload pictures. Your pic will appear together with a cropping frame. Adjust that frame (with your mouse) to the desired size, fill in the rest of the page and click 'Upload' at the bottom.
Have a look at your page, or the dashboard, to see if your picture was uploaded successfully!
Success!