Cameronian,
It might be possible to use the image from the old PC on a new one, but I wouldn't bet on it.
It will be preferable to backup the data independently of any system images created, so that it can simply be copied across to a new PC.
It would be straightforward to create a blank partition of 20 GB on the new PC (besides the new OS system disk) and restore the old image onto that, and then extract the data as required.
Fundamentally, creating a disk image is different to copying files from one location to another. The image is a disk block by block copy (blind to what the blocks contain), the other is a file by file copy (blind to which blocks are used).
More recent disk imaging software may allow individual files to be recovered from the image, but you would need to verify this with whatever imaging software you have or buy.
SD