Every compressed image makes one basic trade: a smaller file in exchange for either nothing (lossless) or a little detail (lossy). Knowing which trade you are making is the difference between a photo that looks perfect at a fraction of its size and a screenshot with blurry, unreadable text.
Lossless Compression: Smaller, Identical
Lossless methods find patterns in the image data and store them more efficiently — a bit like writing "20 blue pixels" instead of listing blue twenty times. When the image is opened, it is rebuilt exactly.
- Formats: PNG, lossless WebP, GIF
- Best for: text, screenshots, logos, diagrams, flat colours
- Limitation: photos don't shrink much, because natural images have few repeating patterns
Lossy Compression: Much Smaller, Slightly Different
Lossy methods go further: they permanently discard detail the eye is least likely to notice — fine texture, subtle colour variation. The result can be many times smaller than the original.
- Formats: JPG, lossy WebP
- Best for: photographs and complex images
- Limitation: push it too far and artefacts appear; the lost detail can't be recovered
Side by Side
| Lossless | Lossy | |
|---|---|---|
| Quality after compression | Identical | Slightly reduced |
| Size reduction on photos | Small | Large |
| Size reduction on graphics | Good | Good, but can blur edges |
| Safe to re-save many times | Yes | No — quality drops each time |
| Typical formats | PNG | JPG |
What Artefacts Look Like
When lossy compression is too strong, you'll notice:
- Blockiness — square patches, especially in smooth areas like sky or skin.
- Halos — faint ripples around sharp edges and text.
- Colour banding — smooth gradients turning into visible stripes.
- Smudged detail — hair, grass and fabric losing texture.
A good rule is to compress, then zoom to 100% and compare with the original. If you can't see a difference at normal viewing size, the setting is fine.
Choosing the Right Approach
| Image | Recommended |
|---|---|
| Phone photo for a form | Lossy (JPG), moderate setting |
| Product photo for a website | Lossy (WebP or JPG) |
| Screenshot with text | Lossless (PNG) |
| Logo or icon | Lossless (PNG or WebP) |
| Image you'll keep editing | Keep a lossless original; export lossy copies |
Compressing in Practice
Image Compress reduces file size while aiming to keep images looking the same. A few habits help:
- Always keep the original. Compress copies, not your only file.
- Compress once, from the original. Compressing an already-compressed JPG compounds the loss.
- Resize before compressing if the image is much larger than you need — fewer pixels means a smaller file with no quality penalty at the display size.
- Check at 100% zoom before uploading anything important.
The Same Idea in PDFs
PDF compression uses the same principles on the images inside a document. That's why a scanned PDF can shrink dramatically while text-only PDFs barely change — see why are some PDF files so large and screen vs ebook vs print compression levels. For choosing between formats in the first place, read JPG vs PNG vs WebP.
