Here's a simple rule for text and visual media:
1. Did you create it? If "yes", you are okay (unless you assigned it or had an obligation to assign it to someone else).
2. If not, then do you have permission from the person who created it or to whom rights in the material are assigned? If "yes", you are okay.
Otherwise, if it is material you did not create, and you do not have permission from the creator or the rights owner, then, no, it is not "100% legal" for you to use that content.
There are some oddball exceptions, such as if it is content produced by the US government (in which copyright cannot be claimed), but the general answer to your question is "of course not."