Lossy and lossless compression
Lossless compression rewrites a file more efficiently but keeps every pixel exactly the same. PNG works this way. Lossy compression throws away detail the eye is unlikely to notice. JPG and WebP can work this way, and it is where the big savings come from.
Because lossy compression discards information, the result cannot be restored to the original. Always keep a copy of the original file, and compress from that original rather than from an already compressed copy. Each lossy save starts from the pixels left by the last one, so damage can build up even when every single save looks fine.
Metadata also adds weight. Camera files often carry embedded previews and location data. Removing it usually saves a small amount, and it protects your privacy when you share the picture.
Which quality setting should you choose?
For photos, a quality between 75% and 85% usually looks almost identical to the original while being a fraction of the size. Going above 90% mostly adds file size, not visible quality. Going below 60% often starts to show blocky areas and blurred edges, especially around text and sharp outlines.
- Photos for the web: 75-85%
- Photos you may edit again: 90% or higher
- Small thumbnails: 65-75% is often enough
These are starting points. Busy, detailed photos tend to need a slightly higher setting than smooth ones such as a clear sky.
Reduce the dimensions first
A phone photo can be 4000 pixels wide, while a blog column rarely shows more than 1200 pixels. Resizing to the size you really need often saves more than any quality setting. Resize first, then compress.
Pick the right format
Photographs compress best as JPG or WebP. Logos, screenshots and graphics with flat colours and sharp text are better as PNG or lossless WebP. Saving a photo as PNG usually gives a file several times larger than JPG with no visible benefit.
A worked example
Imagine a 4000x3000 phone photo that is roughly 4 MB as a JPG. It will be shown in a 1200-pixel-wide article column. First resize it to 1200x900, which removes most of the pixels. Then compress at around 80%. The result is often a few hundred kilobytes or less, and on screen it looks the same as the original. Exact numbers vary with the picture, so treat this as an illustration, not a promise.
Now take a screenshot of a web page, around 1920x1080 pixels. Saved as PNG it keeps the text crisp, but the file can still be large. Resizing to the size you need and, if the content allows, converting to lossless WebP often helps more than lowering quality.
Step-by-step workflow
- Keep the original file somewhere safe.
- Decide the largest size the image will be displayed at.
- Resize to that width, keeping the proportions.
- Compress at about 80% and look at the preview.
- Compare with the original at 100% zoom.
- If you see artefacts, raise the quality a little and try again.
- For a batch, test your settings on the most detailed image first, then apply them to the rest.
Gradients and skies deserve a close look, because lossy compression can turn smooth colour changes into visible bands or steps.
- Download the result and use it in place of the original.
Common mistakes
- Compressing the same JPG again and again, which adds a little damage each time.
- Choosing 100% quality, which makes files large without a visible gain.
- Skipping the resize and relying on quality alone.
- Saving a photo as PNG for the web.
- Overwriting the original so there is nothing to go back to.
- Judging quality only at a zoomed-out view, where problems are hidden.
- Compressing graphics with flat colour and text as lossy JPG, which often leaves visible fuzz around the edges.
Quick checklist
- Original saved separately
- Dimensions match where the image is shown
- Format suits the content
- Quality around 75-85% for web photos
- Checked at 100% zoom
Do it privately in your browser
DoneImg compresses images inside your browser, so the files are not uploaded anywhere. Open the compress tool, add your images, choose the quality, check the preview against the original and download the result. If the picture is still very large in pixels, use the resize tool first and then compress.
Is compressed always lower quality?
With lossy settings some data is removed, but at sensible settings the change is usually not visible. Lossless compression removes nothing but saves less.
Will my files be uploaded?
No. The work happens on your device in the browser, so the pictures stay with you.