(I'm no expert or something at all, but here we go. This is just my opinion, so if I'm wrong at something...yeap, I am a human... a human that can't draw)
Be sure, my friend: most of the time you'll spend in your zine production will be curating CC images and public domain paintings. I truly believe there is a bloody awesome #ttrpg zine inside every CC old illustration.
If you wanna dive deeper on this subject there's a free website teaching you everything! Actually is teaching lawyers, but if they can learn, we can too!
This is my favorite part. It's risky to choose a theme before looking if it has images to the zine. So if you ask me, I would look for stories that are awaiting to be created inside the images. Make a visual research first and create on top of that.
There's a great way to find good public domain maps to use in your zines.
Look for architecture books (or house plans) in archive.org