Congratulations on wanting to make your hubs the best they can be. Photos can be found anywhere on the web. Until you become more familiar with taking images from the web, try places like Flickr or photostock. Be careful to read any warnings regarding use. Some photos have a copyright. This will be displayed on the same page as the photo if this is the case.
I have found that by googling images for a hub I want to highlight (such as: food forests, water crises, etc.), I find many wonderful images. I usually take images from smaller bloggers or websites that do not generally use copyright material and I give credit to the site I took the photo from.
I find that no one (and that includes me on my blog) minds if you use the photo as long as you give credit where credit is due AND are not using it to make money with. Income from sites from Hubpages doesn't count because the small income generated is for the hub NOT the photo.
If you have any doubt about the use of an image, don't use it. Err on the side of caution.