It's not as much that they take your copyright as much as they stipulate that you give them the right to do whatever they want with your image. For example Imgur's terms of service states "With regard to any file or content you upload to the public portions of our site, you grant Imgur 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 distribute any such file or content. To the extent that you delete any such file or content from the public portions of our site, the license you grant to Imgur pursuant to the preceding sentence will automatically terminate, but will not be revoked with respect to any file or content Imgur has already copied and sublicensed or designated for sublicense. Also, of course, anything you post to a public portion of our site may be used by the public pursuant to the following paragraph even after you delete it."
Basically if you upload anything there, then they can use your work for anything, including advertising, without providing you any compensation for its use. To top it off, you can't even revoke that permission even if you delete it. So far, every site I've come across has that or a similar notice in their terms of service.
I plan on using Patreon, even with this stipulation only because you can be paid for your work by other users.

Hi there, welcome!