To use this script, select the mounted volume in the Finder (or in the "Devices" section of the Finder sidebar), and trigger this script via your favorite macro app instead of ejecting the disk image normally.
tell application "Finder" set selection_list to selection if (count selection_list) < 1 then display dialog ¬ "Please select a volume mounted from a disk image." with title ¬ "No Selection Found" with icon stop ¬ buttons ["OK"] default button 1 return end if set my_selection to item 1 of selection_list set my_kind to kind of my_selection set my_name to name of my_selection if my_kind is not "Volume" then display dialog ¬ "Please select a volume mounted from a disk image file." with title ¬ "Selection is not a Disk Image" with icon stop ¬ buttons ["OK"] default button 1 return end if set volume_list to paragraphs of (do shell script "hdiutil info | grep ^/dev/disk | grep -o '/Volumes/.*'") set source_list to paragraphs of (do shell script "hdiutil info | grep ^image'-'alias | grep -o '/.*'") set match_found to false repeat with v from 1 to (count volume_list) if "/Volumes/" & my_name = item v of volume_list then set match_found to true exit repeat end if end repeat if match_found is not equal to true then display dialog ¬ "The selected volume does not appear to be a Disk Image." with title ¬ "Could not find Disk Image" with icon stop ¬ buttons ["OK"] default button 1 return else set my_source to POSIX file (item v of source_list) as alias move my_source to the trash eject my_selection --reveal my_source end if end tell
Could you simply have a search that looks for old .dmg files in your Downloads folder? Yes, though I often forget to open and install .dmg files right away, so I prefer linking the delete action to the eject.
[kirkmc adds: This works for me if I select a volume in a Finder window, but not in the sidebar. This hint was submitted before Mountain Lion, so perhaps there's something that needs to be tweaked for 10.8. I figured it is worth posting so you all can find what to fix; I'll update the script in the hint if someone provides a solution in the comments.]

