Legally, what you do for your own private consumption [like a screensaver you DON"T pass on to friends] within your household or what you write or quote or include in a physical letter or a single email to a friend---all fair use. Posting a picture or a poem on your website, however, is not fair use, since that is given to be "publishing," because it then goes into the possession of multiple other people who can download it or take it without recompense to the author/artist. It's basically the dividing line between the use you'd make of a book or painting if you had purchased a copy of it---fair use---or the use you make if you're illegally multiplying it for the use of multiple people.
If you'd like to prevent this sort of thing, make up a little 'stamp' you include on your online photos that says "[copyright symbol] [year] [your professional name or studio.] If you overlay it onto the corner of the photo and they remove that, it's pretty clear evidence of bad intent with knowledge of what they're doing. If they just use it, you've got proof, and they've got no permission. If you're selling your photos elsewhere, you've got provable damages, and can sue them.
:lol:
