Most of us treat images as a design problem, paste a URL from a stock site into a template, and move on. That is the part of the stack that quietly creates both legal exposure and slow pages, so here is what actually matters about free images from an engineering point of view.
Free Is Three Different Licenses
The word free covers three situations that are not the same thing. Public domain and CC0 let you do anything with no conditions. Attribution licenses let you do almost anything as long as the credit line exists and is correct. Non commercial and share alike licenses carry conditions that are legally binding even though the download cost nothing, and share alike in particular can reach into what you wrap around the image.
There is a fourth question that has nothing to do with the license file. If a recognizable person, a piece of artwork, or a branded product appears in the frame, you may need a release regardless of what the photographer granted you. The photographer can license their copyright. They cannot license someone else's face.
The Metadata Nobody Automates
The failure mode is not picking the wrong license, it is being unable to prove which license you were given. Stock sites reorganize, accounts get deleted, and terms pages get rewritten, so two years later the evidence is gone and the burden is on you.
Treat it like any other dependency. When an image lands in the repo, drop a sidecar file next to it holding the source URL, the license name and version, the uploader, the download date, and an archived copy of the terms as they read that day. It is the same instinct as a lockfile: pin what you actually got, not what the page says today.
Image Prep Belongs In The Build
The other half of the problem is delivery. Serve AVIF or WebP with a fallback, generate the responsive sizes at build time rather than shipping one 4000px original, set width and height attributes so nothing shifts while loading, lazy load anything below the fold, and write alt text that describes the image instead of repeating the filename. All of that is mechanical, which means it belongs in CI, not in a checklist someone follows by hand on a Friday.
The Takeaway
Images are the one asset class most teams handle with less rigor than a dependency, and both failure modes, a takedown notice and a two second render, arrive long after the decision was made. The libraries worth using, the free AI generators, the license rules and the prep steps are collected in this guide to free images, which is a good place to point whoever on your team is pasting stock URLs into templates.