Check out the HubPages learning center guide on "A Guide to Proper Image Use on HubPages". You can find it by clicking on the "help" link above any HubPages page, then click on the Learning Center link that you see. This will take you to the Learning Center, which includes many useful and helpful information to help you understand a lot of things about HubPages.
The guide on proper image use includes a lot of websites where you can find pictures that are either in the Public Domain or have Creative Commons licenses. Pictures like these are the ones that you can freely use. The guide has more information on how you should cite your sources when you use these images.
Once you have your photos, you can put them on your hub by using the photo capsule. Using the photo capsule, you have three choices. You can upload a photo from your computer to your hub. Or you can import a photo from another site by pasting the URL in the capsule and clicking import. If you already have photos in HubPages, just click My Photos and you will see all of the photos you have uploaded or imported. Hope this helps.