Is it ok to use other people’s photos as reference?
It depends. Just like you own the copyright to your art, a photographer owns the copyright to his or her photos.
If you copy all or most of someone else’s photo without permission, this could be a copyright violation. There is no rule that says “if you change it X%, then it’s ok.” In U.S. courts, the test is if a reasonable observer could look at the original and the copy side-by-side and tell that it is a copy. It is ok to copy someone else’s photo as much as you want if it is only for your personal study. It is considered a courtesy to acknowledge your source if you then show that work to anyone.
Here are some examples of ways artists can use others’ photos as reference:
Using individual, generic parts of a photo. Ex. A tree, hills, clouds.
Using individual, specific parts of a photo. Ex. The Empire State Building, a Jeep.
For historical research. Ex. Looking at pictures of WWII uniforms to get the design accurate.
Gathering multiple photos of a subject without using a specific one. Ex. Looking at many photos of lions to see how they are built and how they move.
Using multiple photos for general inspiration. Ex. Gathering photos of different kinds of machinery in order to get inspiration for your own machine design.