Finding a way to designate source of image

I have two sets of images covering the same time period.  One slides and the other prints.  They were scanned by a service and I have thumbnail catalogs of each.  I would like to somehow designate the source of the image (slide or print).  I could put a code in the file name or perhaps in some metadata field.  It would nice to be able to search for them, which would mean metadata.  Does anyone have a suggestion?