These can be images that you get from:

Your own personal camera using droplr or snagit
Screenshots that you take
Wikimedia.org
Sxc.hu
CC0 (Create Commons Zero Sites, free to use in any way you like)
Pixabay.com
Unsplash.com
StockSnap.io
KaboomPics.com
Magdeleine.co
Paid photos (that you buy via sites like Fotolia.com)
Just remember that content you get from website "royalty free" websites is not free to take. You need to read the permissions for each image that you use as each one will have different rules.