Reducing photo file size without visible quality loss
Start from the largest source, choose JPG for photos, and re-encode once. Small, careful steps shrink files while keeping quality that viewers cannot see degrade.
Start from the best source
File size starts at the source. If you have a high-resolution original, convert from that master instead of repeatedly re-saving a copy that has already been compressed.
Every lossy re-save discards a little more detail. Converting the original once to a well-chosen output keeps quality high and size low.
JPG for photos, PNG for graphics
For photographs and images with smooth color, JPG gives the smallest file at a given quality. PNG keeps every pixel but costs several times the size, with no visible benefit for most photos.
For screenshots, logos, or images with text and flat color, PNG keeps the edges sharp and is often the right trade-off.
One careful conversion beats many
Convert once to the format you need and check the result at full size. If the file is still too large, reduce the pixel dimensions rather than compressing again.
A 3000-pixel-wide photo uploaded to a site that shows it at 800 pixels is mostly wasted data. Resizing to the display size is the fastest way to a small file with no visible loss.