It's possible to a worm, self replicating type of malware, virus spread across from one drive to another attempting to recode various files as it goes along including an image you may on an external drive. The incident for this of course would be either downloading directly to or copying files over to the separate drive and cilcking on something for it to see it made active there however.
Most viruses and other malwares are aimed at the OS however with tricksters hiding trojans and rogue programs typically in the user account sub folders. If a bug is present without necessarily having infected the system at the time an image is made it would simply be included in the image to be restored later when it could then come to life and be a problem.
In the last 2 1/2yrs. I haven't seen any one bad image as far as being corrupted when made either with the backup option in 7 or by a 3rd party software(Acronix True Image) while not ruling out the possibility of running into one sooner or later. The idea of having a current on onhand is in case of things like a virus infection where the main drive would then need a complete wipe you would still be able to restore everything in about a half hour's time over spending how many hours reinstalling the OS and all programs, Windows updates, etc.?
The smart move especially when only having one main drive in portable unit and a second external for storage and backup would first safe guarding the things you wouldn't want to lose by backing things up as you go along manually! In the event the main drive is trashed by malware or fails you have kept everything updated on the external HD! Even if you elect to go with a new larger drive your files are still there on the external.
As for image restoration the process will automatically reformat the drive the image will be restored onto meaning it gets wiped clean first before unpacking the archived OS, programs installed, and files. This is why a current image will be a larger help when not keeping up with manual backups of the latest new files you have since that will contain most if not all of them.