As you may have seen I do not put watermarks on my photos. I find that to be a bit time consumming, and besides I don't have a logo ... yet.
You can use my photos if you link back to my blog or if you ask for permission beforehand. This blog requires a lot of work and time, so I would not like for someone else to take credit for all that.
If you want to use a photo from a guest of mine, you should ask them for permition.