Since you've already worked with Marvel on providing them with art for sale, why not ask them if you can have permission to do some sales of posters from the series? I'm not sure about the others, but you never know, they might be just fine with it, though you might have to pay a certain percentage of the sales to them. Either way, I know I'd buy some of them for sure.
It's a tricky thing, while it is illegal, I'm not sure any company would take legal action against you. Sounds bad, but think of it this way. Tons of artists go to conventions and while they do sell original material, tons of them also sell commissions and prints from characters they don't own and the company's don't care.
I know personally, I bought the Avengers box set (got the temp dvd through Amazon) & I took advantage of the poster offer from the DVD too. So I already have those out of the way, but depending on the cost, I wouldn't hesitate to pay you money to sign the Avengers prints and throw in some other signed prints from GhostBusters, Jurassic park and maybe a few others, and maybe I'd just pay you more than shipping, cost of labor to sign for the other prints.
Step Two - Find out how much you have to pay copyright owners.
Step Three - KICKSTARTER.
I know personally, I bought the Avengers box set (got the temp dvd through Amazon) & I took advantage of the poster offer from the DVD too. So I already have those out of the way, but depending on the cost, I wouldn't hesitate to pay you money to sign the Avengers prints and throw in some other signed prints from GhostBusters, Jurassic park and maybe a few others, and maybe I'd just pay you more than shipping, cost of labor to sign for the other prints.