So I have an image I’m using as my Composition Mode background. It was fine at first (once I figured out where to find the settings for Composition Mode in 3.0), but now it’s appearing as tiles instead of filling the entire background the way I want and I can’t figure out why that changed or how to change it back. Help?
I can’t help with your tile question, but I’d be grateful if you’d share how you changed the image in the first place. I can’t find it in 3.0. Thanks in advance!
Edited to say: found it! This should answer your tiling question as well. Go to Project Settings>background image. https://forum.literatureandlatte.com/t/can-we-please-have-the-option/38714/1
It sounds like you have found your way to where one would set up a texture (like paper or wood for example), in the Appearance: Composition Mode: Colors preference tab.
Setting a backdrop on the other hand is a project specific setting (you might want an image in one project but not another). At this point most project settings have been relocated to Project/Project Settings…. Check out the “Background Images” section.
This is made note of in the What’s New appendix of the user manual. Might be a good idea to go through that at some point, and certainly so when running into something like this where stuff has moved around. I did try to anticipate most points of confusion, but if I missed something let me know. 
Just to let you know, I originally went to Scrivener > Preferences > Appearance > Composition Mode.
But according to the other great commenter here, I should have gone to Projects instead anyway. 
Thanks!
That is indeed what I found my way to.
Helped a lot!
Great, thank you so much! I’m that person who was like “Nah, I know how to work Scrivener, I don’t need to read any manual!” Obviously I was wrong, hah.