Shrinking a batch of photos without uploading them
Free photo compressors are everywhere, but most cap out at a handful of free images before asking for payment, and nearly all of them work by uploading your file, processing it on a server, and sending it back. Run fifty photos through one and the upload-and-download time adds up on top of the actual compression.
Doing it in the browser removes that wait entirely. The photo gets redrawn onto a canvas and re-encoded as JPEG or WebP, so nothing leaves your computer, and there's no cap on how many you can queue at once.
JPEG or WebP
JPEG is the safe default. It opens everywhere — old software, print shops, whatever the recipient has. WebP produces a smaller file at the same visual quality, but some older software still can't open it. If it's going on the web, WebP wins; if you're handing the file to someone else or sending it to print, stick with JPEG.
1920 on the long side covers most screen use
For anything viewed on a screen, 1920 pixels on the long side is enough for nearly every case — most monitors are around 1920 pixels wide, so anything bigger just adds file size without adding anything visible. Unless the photo is headed to print or a big zoom-in, there's little reason to go past that. For a blog post or a chat, 1280 is often plenty.
Re-encoding never makes a photo better than the original
Shrinking is re-encoding, and re-encoding can't add detail that wasn't already there. Run an already-compressed JPEG through compression again and you mostly lose a little more quality for barely any size difference. If a photo's already small — especially one that's been saved more than once — it's usually better left alone.
If the original was already compressed efficiently, re-encoding it can even make the file bigger. When that happens here, you get the original back instead, with both file sizes shown so you can see it wasn't actually smaller. If a photo doesn't show as shrunk, that's the tool telling you it was already about as small as it should be.