You can dismount by running the ShadowProtect UI and select Dismount Backup Image under Tasks. I believe this is an install issue where something isn't registered properly. You could try to rerun the installer, but it'd probably just be easier to manually re-register the shell extension DLL which is responsible for adding the "Dismount..." and "Quick Dismount" menu items to the File Explorer context menu (which pops up when you right click on a mounted image's drive). To do this:
1) run cmd.exe (the command-line / DOS box shell)
2) register the shell extension with the following command:
regsvr32.exe "\Program Files\StorageCraft\ShadowProtect\sbimgmnt.dll"
You should then see a small dialog box that states that the registration succeeded. You'll have to reboot (so that Explorer.exe restarts) for this change to take effect. Or, you can just go to taskmgr.exe (CTRL-SHIFT-ESC) and kill all "explorer.exe" processes, and then while you're still in Task manager go to its run dialog and enter "explorer" (without the quotes) into the run dialog's edit box and run that command - this will launch Explorer again so that you have your task bar.