sorry I don’t use Safari or a Mac. I only ever load as .jpg. I would suggest that you reduce the size of the images. Even if under 4mb because 4mb really is very big. Also it is better to have your main photo square, I checked one of yours and saw it wasnt.
Are you using photos? If so you can reduce the size under the edit. Do not have that problem but my items are smaller than paintings.
What do you use to take the images, iPhone adjusts automatically but if a camera could be in the settings.
If it’s another nginx coding problem (like the ‘header or cookie too large’ one we kept getting a few months back that stopped us getting into our shop dashboards) it may be fixable by clearing your browser cache. You should also drop an email to Support with as many details as possible about your computer, browser, operating system, and what triggered the error - this will help them to track down the source of the problem and hopefully fix it on their end!
ETA I’ve just added a new listing as a test and the photo upload worked fine, so hopefully they’ve already fixed it. (I’m using Firefox and MacOS Sonoma.)