While in Repair Mode, you have an option to open a Command Prompt. From there try the following commands:
Mountvol
(This command will show you all available drives, including your external disk)
dir Q:
(Replace Q: with one drive letter at a time from the letters that the mountvol command showed you, e.g.
dir D:
dir E:
etc.
until you see a folder "WindowsImageBackup" on the screen. Let's assume it is F:\WindowsImageBackup.
dir F:\WindowsImageBackup /s
(This command should show you the contents of the image folder, with files totalling more than 10 GBytes)
Now perhaps this image folder is missing, incomplete or inaccessible.
I have to add that I am disappointed with the native imaging facility under Windows 7. It is temperamental and its functionality is extremely limited. I recently bought myself a copy of Acronis TrueImage 10 and its is to the Windows imaging tool like a fully functional motor cycle is to a child's tricycle. Other brand imaging tools are just as functional - just make sure that they are certified for Windows 7.