I have been thinking about this a lot with my own blog lately, and I think it really depends on what your subject matter is. I should say first of all that if I do use other people's work, I always credit the source. I do a lot of pieces that feature artwork and music, so naturally those things are going to use outside images a lot of times. But if I do a post that is about something like a DIY project or a recipe, then I will definitely take my own photos. The post is more personalized that way, and it gives readers a more authentic connection to you and what you have to say.