Browser-based vs. server-based compression, and how to pick the right free tool for what you're actually doing.
There are dozens of "free image compressor" tools online, and most of them do roughly the same core job — but the details differ enough to matter depending on what you're doing.
Three things separate a good free compressor from an annoying one: whether it uploads your file to a server or processes it locally, whether you get a live preview before committing to a quality level, and whether there's a limit on file size, batch count, or daily usage that nudges you toward a paid plan.
Tools that use your browser's own Canvas API — like SNTools' Image Compressor — process the file entirely on your device. Nothing is sent to a server, which matters if you're compressing anything sensitive, and it also tends to be faster since there's no upload/download round trip. The tradeoff is that very large batches or extremely high-resolution images can be slower than a dedicated server-side tool, since your own device is doing the work.
Tools like TinyPNG or similar services upload your image to their servers, compress it there, and send it back. These often use more advanced compression algorithms and can handle batch processing well, but you're trusting a third party with your files, and most free tiers cap how many images you can process per month.
For one-off personal photos — privacy-first, browser-based tools are usually enough and skip any upload limits. For bulk website asset optimization across hundreds of images, a server-side batch tool or a build-pipeline plugin (like a WordPress compression plugin) makes more sense. For hitting an exact file-size requirement, like a passport photo capped at 100KB, look specifically for a tool with a target-size mode rather than just a quality slider, since it saves you from manually guessing.
If privacy or speed matters more, pick a browser-based tool. If you're processing large batches regularly, a server-side or plugin-based tool will save time. Either way, always preview the result before downloading — file size isn't the only thing that matters.