Just to be completely clear, when I say "create 100% yourself", that does not include taking some render or whatever that someone else made and then making edits to it. Editing something a person didn't have permission to use in the first place does not give them the rights to use it. Some people tend to think that just because they put in a lot of time or effort in Photoshop, that it somehow allows them the rights to use whatever they want, which simply isn't true.
There's also a problem with some sites out there that offer "render" images as free for anyone to use, but they don't have any right or license to actually do that. There's a lot of art stolen from artists here and elsewhere that get put up on render sites that are breaking copyright laws. So always ask the original artist if you can use their stuff first if they don't make it known that their stuff is free to use. If you can't find them or can't get an answer, assume that you don't have permission.
This is not a political discussion. If you have questions about the site, read the +faqs or contact the +help desk.
The short answer is that you are only allowed to use things you either create 100% yourself or have expressed permission to use anything you borrow from someone else. That means if you don't have permission from someone to use a "render", you are not allowed to submit it here. Also, do not confuse seeing things other people have done as permission to do the same thing. It is a very big site and not everything can be dealt with or gets reported as quickly as we'd all hope.