I have stumbled across one of my pics on someone else's webpage before...it's a bit...wierd. First I thought, "Hey, that photo looks familiar, where'd I see it before?", then it was "Wait a second - that's MY picture!"
I would have no problem with someone using my photos, provided:
1. I am asked beforehand!
2. Proper credit is given
3. They're not selling the pics 
It's not just a matter of politeness; someone taking photos and in essence 'stealing' them for their webpage is in violation of copyright. I think some people are just unaware of that though. I remember someone on one of these forums stating that 'anything on the internet is in public domain', which is patently false.
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-audri
Webpage/Pics