How image compression actually shrinks a file
A photo from a modern phone often weighs 3 to 8 MB. Most of that is detail your eyes cannot pick out on a screen. JPEG and WebP compression throws away some of that detail in a way that is hard to notice, and the file gets much smaller.
This is called lossy compression. The quality slider controls how aggressive it is. At 80 percent, a typical photo drops to roughly a quarter of its original size with no visible change at normal viewing distance. Below 50 percent you start to see blocky patches and smeared edges, especially around text and sharp lines.
PNG works differently. It is lossless, so the pixels stay exact and the savings are usually small for photographs. PNG is the right choice for logos, screenshots and flat graphics, not for pictures of people or landscapes.
Picking a quality setting that fits the job
For a website or blog, 70 to 80 percent is a good starting range. Pages load faster and the pictures still look clean. For a photo you plan to print or crop heavily later, stay at 90 percent or above and keep the original file as a backup.
If you need a specific file size, such as an upload limit on a form, use the target size presets on the dedicated pages for 100 KB, 50 KB and 20 KB. They search for the right quality automatically instead of making you guess.
Resizing helps more than most people expect. A 4000 pixel wide photo shown in a 800 pixel wide blog column wastes most of its pixels. Scaling it down to 1600 pixels before compressing can cut the file by another half.
What happens to metadata and transparency
Photos carry hidden EXIF data: camera model, date, and often the GPS location where you took the shot. Because each image is re-encoded here, that metadata is dropped by default. That is useful when you post pictures publicly and do not want to share where you live.
Transparency is only kept when the output format supports it. PNG and WebP can have transparent areas. JPG cannot, so a transparent PNG converted to JPG gets a white background.
Why browser-based compression is safer
Many compressors send your files to a server, process them there and send them back. That is fine for a holiday snap but uncomfortable for ID cards, receipts or client work. Here the compression code runs on your own device, and no file is uploaded at any point.
Each file can be up to 25 MB, and you can process up to 20 at a time. On a slow phone, large batches may take a little longer, but nothing is sent over the network while you wait.