Resize an image without installing anything
"Make this image smaller" means two different things. 4032×3024 is a size and 3.8MB is a weight, and the first question is which one somebody is asking you to reduce.
Cut the pixels and the file size comes down with them. Cut the file size alone and the pixels stay where they were. So in almost every case you start with the pixels and let the weight follow. Squeezing quality out to save a few more kilobytes is a second step, if you need it at all.
Pick a width based on where it is going
You can ignore the height. Lock the ratio and it follows the width on its own.
| Where it is going | Width |
|---|---|
| Sending in a chat app | 1280px |
| Body of a blog post or forum thread | 1200px |
| Full-width background on a site | 1920px |
| Profile picture | 500px |
| Attaching to an official form | Leave the original |
A phone photo is around 4000px wide. A forum post needs 1200. Just setting the width to 1200 usually drops the file under a tenth of what it was, and you can't see the difference.
There are only three formats worth knowing
JPG for photographs. Landscapes, people, food. The catch is that it loses a little every time you save, so opening, editing and saving over and over will not come back to the original.
PNG for screenshots, anything with text in it, and anything that needs a transparent background. Nothing is lost, but a photograph saved as PNG can be several times the size of the JPG. Save a screenshot as JPG and the edges of the letters go grubby, so leave those as PNG.
WebP lighter than JPG at the same quality. Google made it and every current browser reads it. Older software and some upload forms still reject it, though. WebP for things going on the web, JPG for files you're handing to another person, and you'll not get caught out.
You cannot go the other way
Stretch a small image up and it goes soft. Quality that was never captured does not appear. Take something 200px wide up to 1000px and all you have is a blurry 1000px.
Short of finding the original again, there is no fix. There are AI services that will upscale for you, but what they are really doing is inventing the missing detail convincingly, so it is not something to use on a person's face or on text.
One thing about online tools
Most sites that compress images for you upload your photo to their server. They'll say it's deleted afterwards, and it probably is, but if it is a photo of your ID or a signed contract, there is no reason to send it anywhere in the first place.
If the work happens inside the browser, the file never leaves your computer.
Dropping an image into the image board, or pasting it (Ctrl+V, or ⌘V on a Mac), gets you the width, height and ratio for free. Cropping, resizing with the ratio locked, and saving as PNG, JPG or WebP all happen in one panel, and the new file size is printed alongside.
None of that involves a server. Nothing to install, no account to make, so try it on one photo and judge for yourself.