This is simply list of different useful resources I use, or have found to be useful, along with some great tutorials. I hope this is useful for whoever reads it!
Royalty free photos:
(These photos are completely free, and are therefore ideal as references for artworks which are sold and/or used commercially.)
The largest website for free images. This site has literally everything, from cars to foxes. You need to get an account, but this is completely free.
As the name says - free images. It is not very large, but they have some HQ photos. You'll need an account, but this is free.
Public domain images. No account needed. There are not that many photos, and the quality might not be so good. Still, there are some hidden gems.
A lot of good, free stock photos.