I have my image resources in the various
New -> Directory
New -> Folder
How can I create usable sub-folders in my mipmap folders?
You can't. Resource directories of any time,
mipmap or otherwise, cannot have subdirectories.
what is the correct way to include (and organize) hundreds (or thousands) of image resources in a project? Is the only way to do it really to have them all in a single directory?
If you are going to go with resources, then yes, though I would put them in
mipmap is mostly for launcher icons.
the mipmaps are special because the app will display images of a different resolution depending on the device resolution
drawable resources support resource resampling at runtime, but it is based on density, not resolution.
As ByteHamster notes, you can put whatever sort of directory structure you want in
assets/, though you will need to put the appropriate values into
BitmapFactory.Options to help with the resampling based on density.