Another option that hasn't been mentioned here is to use "buddy files". This low-tech solution has advantages because the text is clear and editable, it is searchable, and it will not get wiped by some photo-editing s/w like the Exif/XMP meta-data. It is also possible to add keywords or search terms of your own choosing, and even full transcriptions.
A "buddy file" (or "sidecar file") is a separate file that shares the same name, but with a different file-type (i.e. different file extension). For instance, having a tony.jpg image and a tony.notes text file. This may sound almost too trivial, but the difference is when they are linked by a bit of software.
One free Windows example of such software is MetaProxy, which works with all image types and many document types too. It will open the image (or document) in your usual registered application, and then either display the buddy file by the side of it or overlaid on the bottom third of it. It also supports image collections, and there is also a Mac equivalent.