If any external resources are used, other than stock photography, they are stored along side client files for a project. I am not one to reuse the same external resources over and over and over for different clients, except for stock photography in some instances.
For stock photos, I store them in a separate alphabetical directory with subdirectories for the source of the photo i.e.:
File titles are altered... [ myDescriptiveTitle_sourceAbbreviation-imageNumber ] I can then go to the source web site, and search for the image number if I need the license again.
And note that for Stock photos, most services will allow you to view you account and download again if needed. So, often there may not be a need to store raw images locally. However, it's exceptionally rare I use a stock photo in it's downloaded form without editing it.