Try the following:
Mount the disk image so that there is a disk icon on the Desktop, and then launch Terminal (in /Applications/Utilities). Copy-paste the following line into the Terminal window, and then type a single space. Do not type <Return> yet:
sudo mdutil -i on
After typing the single space, drag the mounted disk image icon from the Desktop into the Terminal window, click inside the Terminal window, and then type <Return>. Enter your admin password at the prompt (it will not echo on the screen), and again type <Return>.
Hopefully that will enable Spotlght indexing on the mounted image. It might take some time to complete depending on the contents.