A picture says a thousand words. More relevant for this context is that a picture just adds visual appeal to the blog/site.
Here are a couple blogs that list several sites where one can obtain royalty-free images.
Personally, I’ve been using Pixabay almost exclusively. It has a pretty good selection of photos. I’ll keep using it ’til I find it harder to find good images. I’m not a fan of hunting around to multiple sites to find the perfect image. Good is good enough for this purpose.