A few years ago I did a calendar shoot for a local newspaper. In thanks, they said they would make a free full back-of-the-calendar thank you ad. Since my wife is a pro-graphic designer we offered to make it. They said "no-no-no, we'll do it" Three months later we get copies of the calendar and surprise! - they had taken photos from my website to make the ad.
The publisher of the paper had the same delusion - "oh, but they're on the web so we just used them." Her ignorance of copyright law blew my mind. I countered asking her how fast would she sue me if I took some copy or photos from her website and used them on one of my advertisements. To make it worse, she admitted her design team often went out on the web to look for photos and it had never been a problem. Beyond that, I held the model releases, not the publication, so legally I could have forced them to pull the back page on all calendars (the assumption being that the subjects in those photos wouldn't want to be on this calendar - even if they agreed to be on my website).
Obviously this was a commercial situation, not personal, but the same rules can apply for so-called personal use, particularly when personal use means posting it on the web for everyone to see.
Fair use as far as the copyright laws of the US can be somewhat confusing but some things are made fairly clear by the US Copyright Office. To put it very simply:
* Fair use can be a gray area except in some very limited circumstances.
* When in doubt, consult an attorney.
* To be safe ask for permission.
Most times, asking for permission is all it takes.
U.S. Copyright Office - Fair Use