Hello - is there any way to combine different picture files in "gallery"?
You mean in albums?
Use a file manager like ES File Explorer to create folders/subfolders (but outside of the folder DCIM ! ) and sort in your pics.
Gallery will show them as albums.
Or do you mean collages of pics?
I use the app KD Collage for this, but there are a lot of apps of this kind in Play Store
I mean albums. Also, is there a way to NOT put stuff in DCIM - should I not put pics in DCIM (I see it when I plug the phone into the computer, but to be honest I don't know what it stands for or does!)
The phone's cam app puts its pictures and videos in DCIM
With your cam's stock app you can't choose an other folder on your storage for the pics.
So you have you use a file manager to shove the pics from DCIM in your new created folders for to show them in the new albums.
The new folders have to be outside of DCIM, if the folders are inside of DCIM the gallery won't show them as albums.
Separate names with a comma.