Photography

...grants them the rights to use your images in pretty broad ways:

"With regard to any file or content you upload to the public portions of our site, you grant Postimages a non-exclusive, royalty-free, perpetual, irrevocable worldwide license (with sublicense and assignment rights) to use, to display online and in any present or future media, to create derivative works of, to allow downloads of, and/or to distribute any such file or content, including embedded (hotlinked) into third-party websites otherwise not affiliated with Postimages."
https://postimages.org/terms

That is why I've used Flickr for a long time. They do not require you to give them rights to your images.

You do not give up copyright with either, but if Postimages sees something they like you've already given them permission to use it--even to "create derivative works" of your images.

Like I said: it may not matter to anyone here, but just FYI if you are a creator.