You can also read in image files (.jpg and .png) as "datasets", as in
afni *.jpg
These can only be opened as Axial images, since they are only 1 "voxel" thick.
You can read directories recursively, as in
afni -R1
which will read datasets from the current directory and all its immediate (1 level deep) sub-directories. You can descend deeper with "-R2" and even deeper with just "-R" -- however, do NOT try "-R" at the top of a large disk, because AFNI will try to read everything, and that is not going to work well.
For much more information, including lots of stuff you will likely never use, see the output of
afni -help
or see
https://afni.nimh.nih.gov/pub/dist/doc/program_help/afni.html