You should never have to tell someone not to use your pictures. Unfortunately, it does happen. A lot.
Recently I discovered (again! Yes, I've had this happen twice) that someone had been taking photos of mine (and many, many others) and using them on a few other blogs they ran. Not just posting them, but using them without any credit to myself or the people who actually took the pictures, and some were used without the permission of the owners of the photos (said photos were not posted publicly on the web). One set of pictures was actually posted on a site where prints of the pictures could be purchased.
Remember, if you take a photo, the copyright to that photo is yours forever, unless you give a copyright release to someone. Don't let people steal your work. And don't ever steal someone else's work! Even when seemingly innocent it's just inappropriate.
Now, were the pictures that were used something of a sensitive nature? No, not at all.
Something that could be sold as stock photos or otherwise used for monetary gain? Sometimes yes, which is the big issue.
My point is this - If you want to repost a photo that you see and like on your own blog, simply ask the owner of the photo for permission to repost. AND ensure that you properly credit the photo and possibly provide a link-back to their blog/site. Everyone wants to receive credit where credit is due.
For anyone out there who posts photos on the web, I highly recommend watermarking them in some way. At least then you will hopefully get credit and stock photo sites, etc. won't buy photos with a watermark. I have many photos I need to go back and watermark. If you notice photos not working from my blog, this is why. It's because I have several things I need to go fix if possible. Unfortunately my main computer is on the fritz and needs repairs, so it may take some time to actually get it done. :-(
There are MANY ways to watermark, from simple apps on phones and free websites to advanced watermarking programs. I use a variety, as I often post pictures on chat forums from my phone, but also do a lot of work from my computer. I do have an iphone app for watermarking (a very simple one called iWatermark) and for doing work on the computer I simply use photoshop. The fastest way is to create a brush in photoshop, so you aren't constantly retyping your text, and you can create a cool brush design if you want to. I used this easy to follow and awesome tutorial from BittBox.
For the record, since it appears it needs to be said, Any photos posted here on my blog, photo hosting sites or anywhere else are not for use on other websites without my permission.