Looking for legal licensed for edit various stock - details further

Basically I'm wanting to mix some things together and see what hits for t-shirt art/graphics
(vague i know)
However unlike many I am seeking legal free for commercial use graphics/arts/illustrations wherein I do plan to do various edits (yes beyond filters)
to create my own unique photocomp.

Various cool illustrations
(some of that I can do myself for a 1 off piece but somethings you just don't have time for 15hr + illustration and photocomping )

Classical art - I.e Where would i go to get the recognizable faces of the u.s.a founders and such.

I am also curious I expect no one here to be a copyright lawyer but to be succient without spending hours seeking legal jargon , how can so many people create cool fan art or use known meme art (i.e the spacesuit) to create graphics with marginally less than 25% alteration for t-shirts (teespring/redbubble etc)and not be sued for infringement?

or is that only a fear for the super popular.

Great design resource

100 Things Every Designer Needs to Know About People (Voices That Matter) Submitted November 26, 2018 at 03:30AM by Fluser8419 https://ift.tt/2BxrC9G