Don't overthink it ;- )
There is a law concept in western world (although primarily in US law, which is stricktest in copyright) called "Fair use" that (wikipedia citation follows): ,that permits limited use of copyrighted material without acquiring permission from the rights holders'.
Although you should seek permission if you feature someone's else artwork 'dominantly' in your 'commercial' artwork, do not become slave to this rule. Finding random stuff on google and using it in minor derivative way is absolutely ok in 90perc. of cases.
Another concept to learn is "Derivative work".