when I double click a disk image it mounts under /Volumes/image-volume-name. I want to change that for a specific image. I know that I can control the directory when mounting from a command line, but I'd really rather have it mount to my specific directory when mounted by simple opening in finder.
you can't if you use plain Apple mounting.
A work round is to create a symbolic link where you want the mount to be and link to the /Volumes/image-volume-name The image will mount under /Volumes but you can access it via the symbolic link path
e.g. in terminal
ln -s /path/where/you/want /Volumes/image-volume-name