you could buy some marvel posters or a comic book and cut out the pictures you want
I will tell you one more way, but it is extremely sneeky.
Go back to walgreens or cvs and look for the self serve machine.
Normally, they have to put a code in, in order for the print to start. So what you do is you buy any picture and memorize the code when they enter it.
Then come back later and print your own photos by typing their code in. I would still pay for them and not just steal them. After you print them, they will likely just let you pay for them as its already done.
If they question you, you could say you just guessed at the password, or didnt know they had to do it and you saw someone else doing it.