Want to hear something funny? Those batman, hulk, spiderman (and MANY others) are actually just a downloadable font put at some crazy large size, put against a "papyrus" type background, and then the character name at the bottom.
I know because I saw images like that online and this past summer I made t-shirts for my family for one of the days that we went to Universal Studios in Florida. I did all sorts of color effects, etc, and even an overlay for each character like he did with the hulk one with a "clear" Bruce Banner in front of a black Hulk.
Captain America font set is called Super Soldier
There is a set of X-Men ones called X-Fighters
Avengers one called Heroes Assemble
Heroez for more heroes.
Galaxy Far Far Away, Forced Reentry, and StarWars Kit for Star Wars
and so on.
I would be reluctant to have him print them at Spoonflower. You need the exact right size, and a border, and the image rotated, etc. Not that it is complex, but there is a chance for failure and you will have to probably pay for him to do it. You would be better off asking for higher resolution images and if you had to pay for those at least you could do whatever you wanted with them.
Just my thoughts, but the guy has to be willing to help either way.