Possible to transfer maps made on roll20 to another persons campaign?

Every now and then I'll make maps in an old campaign I made because I've amassed a large amount of tiles, objects, and other things to make and flesh out what I want to make. I'm the guy my DM asks if he wants help making a custom map, haha. My problem and question is if there's a way to either transfer maps over to my DM from my campaign over to his or if I could be made co-DM on his and use all the artwork I've collected to make whatever he needs.

You cannot transfer maps between Games where you are not the creator on both games.  When you are a GM (as creator or promoted co-gm) in a game, you have access to all of your User Library and Marketplace purchases to create maps with.

So the only way for my DM to be able to use the maps I make is for him to make me co-DM and make them in his campaign then?

Correct, the only way for your DM to be able to use maps that you create in his Games, is for you to create them in one of his Games.

Sucks I can't transfer them or save them in some format; but still, thank you.

I believe there are suggestions in the Suggestions and Ideas Forum around such if you'd like to throw your vote on some