Image compression reduces the amount of data needed to store or transmit a picture. The two broad approaches are lossy and lossless compression.
Lossy compression
Lossy compression deliberately discards information to reduce file size. JPEG is the familiar example. At sensible settings, the removed information may be difficult to notice. At aggressive settings, artifacts become obvious around edges, textures and gradients.
Lossless compression
Lossless compression reduces file size without discarding the encoded pixel information. PNG is a common example. When decompressed, the stored pixel values can be reconstructed exactly.
Why lossless files can still be large
Lossless compression cannot simply throw away subtle variation in a photograph, so detailed photos often remain much larger than a visually similar lossy export. Flat graphics with repeated colors can compress very efficiently, however.
Which should you use?
- Use lossy compression for delivery photos when smaller files matter.
- Use lossless compression for graphics where exact edges or repeated editing matter.
- Keep an original source when you may need to create new exports later.
A common mistake
Repeatedly opening and saving a lossy image can accumulate degradation. A better workflow is to keep a higher-quality master and create compressed delivery copies from that source.