A photo straight off a phone camera can easily be 4-8MB, far more than most websites, forms, or email attachments allow. Here's how compression actually shrinks a file without making it look obviously worse.
How JPEG compression works
JPEG compression simplifies color and detail information in ways the human eye barely notices at moderate settings. It groups similar-colored pixels together and discards some of the finer variation between them. The more aggressive the compression, the more detail gets simplified away.
What quality setting to use
Somewhere around 70-80% quality is the sweet spot for most photos: it usually cuts file size by 60-80% with almost no visible difference from the original. Going below 50% starts to introduce visible blocky artifacts, especially in areas with fine detail like hair or text.
Try it on your own image
Use the free Image Compressor →, drag in a photo, adjust the quality slider, and see the size reduction in real time before downloading.Compression vs. resizing, what's the difference?
These solve different problems. Compression keeps the same pixel dimensions but re-encodes the image more efficiently. Resizing actually reduces the pixel dimensions (say, from 4000×3000 down to 1200×900), which also shrinks file size but changes how large the image can display without looking blurry.
For the smallest possible file at a specific display size, like a website thumbnail, do both: resize first with the Image Resizer, then compress the result.
A note on PNG images
PNG is a different format built for images with sharp edges, text, or transparency, logos and screenshots, mostly. Converting a PNG to JPEG for compression will lose any transparency (it gets filled with white), so for images that need to stay transparent, resizing without format conversion is the better option.