I’ve just double checked, and they do indeed accept images that already appear on other stock galleries. In-fact I personally have images on Dreamstime that are also uploaded to some of the galleries listed above. When uploading images there is an option to sell the image exclusively on Dreamstime but you don’t have to opt for it. You can read about this in this part of the terms and conditions: http://www.dreamstime.com/terms#contributors.

If you personally can’t sell non exclusive images, it might be down to the way that you registered (I’d have to double check to be sure, but I think you can register as an exclusive contributor). If you give it a go and have trouble, let me know and I’ll be more than happy to see if I can run you through the way I did it.

That is not true e.g. on dreamstime you have to accept the term that you donát upload the image to anywhere else for sale… Otherwise really useful.