The Hungry Jpeg is One of my favorite sites for about everything you need for graphic content and they have the best commercial license in the business.*
I use Design Cuts all the time for imagery and they have a very generous commerical license.*
Jessica Johnson who is Creators Couture is a talented designer who shares her talent with us. For using in Adobe products primarily. Her site is filled with tutorials and examples. I love her brushes.*
This is a royalty free site, that is indeed free. However make sure you choose Pixabay images because there some some images that are offered by iStock that are paid to help support the free Pixabay. You may want those, but just be aware.
*I am an affiliate and may earn a small commission if you use my links. There is no additional cost to you. Thanks ~ L.A.