It's very simple: I always indicate in my pictures the license under they are released.

If it is "All rights reserved" you can contact me to ask for permission.

If it is "Creative Commons", in any of its variations, just follow the terms of the license. Normally I choose "attribution" which means, basically, that you have to clearly indicate that the photo you are using is mine, and that's all (REMEMBER, always read the terms of use of each license to be sure about what you can/cannot do).

Anyway, I would appreciate if you just let me know that you honoured me by using some of my humble photos.