#though to be fair I've seen some professionally published books with low-res images in them because people are not learning dpi

[themandylion]

Hi, professional book typesetter/desktop publisher/graphic designer here! I've designed more than a few book covers for print publishing companies in my time and have a few tips/things to add.

Resolution. If you are making book covers for print (or, really, anything, for print), you're going to want your images to be high resolution. The go-to resolution for items like book covers is 300 dpi (dots per inch, sometimes called ppi—pixels per inch). Anything lower and you start to lose quality. (A lot of people are posting images at hella-high dpi as screen resolutions improve and they want their art to look good. This makes it so much easier for unsavory people to steal your art, slap it on a shirt, and have unsuspecting customers buy it without realizing it's super-duper stolen. So. Something to think about, maybe. :) )

Color Mode. Most people creating digital art for display on the web are doing it in RGB (red-green-blue) files. This is also, usually, the color mode for most digital cameras and thus the mode of any photos taken with them. It looks great on a screen because it's specifically how light mixes. If you're designing a cover for print, however, it's best to do it in CMYK (cyan-magenta-yellow-black), since those are how ink mixes. You can change this usually under something like... Image > Mode > CMYK; terms/location may vary from program to program. If you don't do this, when you print out your cover it is going to look dull and sad and not as bright and vibrant as what you designed in RGB.

Photopea.com is an amazing free in-browser graphics program that is very similar to Photoshop. A few of the tools are simpler/have to be approached at a different angle than Photoshop, but as someone who has been using paint programs for 35+ years and Photoshop for 25+ years, it's now my go-to when I don't want to boot up my old computer that has Photoshop installed. (I tried GIMP originally and while I didn't find it difficult to install, I found it a pain to work with and had to fight it every step of the way.)

In addition to those sites shared in the OP, a good place you to find free and Creative Commons images is Flickr. That link specifically goes to the advanced search page because you're going to want to mess with the Search filters—click that, scroll down and change Any license to Commercial use & mods allowed. A lot of U.S. government agencies (like FWS and NPS) have Flickr accounts where they post amazing photos. Any photo taken by a U.S. federal employee in the course of their duty is in public domain however this does not cover photos of people (you need photo releases for those).

When using Flickr to find images, it's always a good idea to check the description (especially if it's not a government account)—Flickr's default when you upload is to put all images under CC2.0 and that isn't always the intention of the photographer. :\ This is really important to keep in mind when using Wikimedia Commons (suggested by in the article linked in the OP) because some of the pictures on Wikimedia Commons were picked up from Flickr by a bot and the Flickr OP has since realized they originally posted with the wrong license and subsequently changed it. If you use Wikimedia Commons for CC pictures, always click back to the original source and double-check the license if it was picked up by a bot.

Too many writers are using generative 'AI' to make their book covers, so I've written a guide on how to make your own cover for free or cheap without turning to a machine.

If you can't afford to pay an artist, you CAN make your own!

I hope this is a helpful overview that covers the basics and points to some free resources.