I'm sorry to hear that you haven't received a response from the Dreamstime forum. Since it's been a while, it might be worth revisiting the issue and reaching out to Dreamstime support again. Sometimes, platform updates or changes in browser compatibility can affect upload functionalities, so it's worth checking if there have been any developments in resolving this issue.
And if you're still looking for ways to optimize your images, consider giving compress jpeg a try. Even after all these years, it's still a reliable tool for reducing file sizes without compromising quality.