Common Image Compression Mistakes and How to Avoid Them

Compressing the same file again and again

The most damaging mistake is repeated lossy compression. Each time a JPEG or a lossy WebP is opened, edited and saved again, new errors are added on top of the old ones. After a few rounds, the picture looks noticeably worse, even if each step used a high quality. This happens when a picture is shared, downloaded and uploaded many times, and when people compress a copy of a copy.

The fix is to keep an original and always compress from it, once, with the settings that you need. If you only have a compressed version, do not compress it harder unless you must.

Using PNG for photographs

Photographs saved as PNG are many times larger than necessary. The cause is usually a default setting in a screenshot tool or an editor. Converting such a picture to JPEG or WebP often cuts the file size by 70 to 90 percent with no visible loss at normal viewing size. Keep PNG for logos, icons, interface screenshots and images that need exact edges or transparency.

The compressor shows the size of each result, so you can see the saving immediately when you switch the output format.

Using JPEG for text and graphics

The opposite mistake is to save text, line art or logos as low-quality JPEG. The compression creates halos and fuzzy edges around sharp contrasts, which makes letters harder to read. For such content, use PNG (or lossless WebP made with another tool), or at least a high JPEG quality.

If the picture mixes photographs and text, such as a screenshot of a web page, check the legibility of the smallest text at the size where the reader will see it, and choose the quality accordingly.

Ignoring the display size

Uploading a 4000-pixel photo to show it at 600 pixels wastes bandwidth and slows down the page. Compressing it with a lower quality while keeping all the pixels is less efficient than reducing the pixels first. Decide the display size, resize to it with a margin for dense screens and then compress.

Do not forget the other direction: enlarging a small image adds no detail and makes the file bigger. If a form requires a minimum size and your picture is smaller, get a better original instead of scaling up.

Going too low on quality

A very low quality setting gives a small file but also visible blocks, banding and smeared detail. The extra savings between quality 60 and quality 40 are modest compared with the visible damage. In most cases, the sweet spot lies between 70 and 85, and reducing the pixel dimensions is a better way to get further savings.

Always look at the result at the size at which it will be used. If you cannot tell it from the original, you may be able to go lower; if you can, go back up one or two steps.

Forgetting transparency and metadata

Converting a PNG with a transparent background to JPEG makes the background solid, usually white, which looks wrong on a dark page. Use WebP or keep PNG when transparency matters. And remember that metadata, including location, is removed in compressed copies made by this tool: this is good for privacy, but it also means that the compressed file has no capture date. Keep the original for your archive.

Color profiles can also disappear, which can change colors slightly for pictures that used a wide-gamut profile. For critical color work, such as professional photography or print, keep the original.

Not checking the result

The final mistake is trusting a setting without looking. Different pictures react differently to the same quality, and a value that works for a landscape may be too low for a portrait. Check each important image, or at least a few representative ones from a batch, at the display size and on a real device.

A minute of checking is cheaper than re-uploading a whole set of pictures later. And if the tool reports that a file is already optimized, accept it for that setting: re-encoding in the browser did not make the file smaller.

A safe default recipe

Keep the original. Resize to the display size. Use JPEG or WebP at a quality of 75 to 80 for photographs, and keep PNG for graphics. Compress once, from the original. Check the result at the display size. Name the files clearly.

If you follow these steps, you will avoid nearly all of the problems on this list, and your images will be small, sharp and consistent.

What to do when a tool says no gain

Sometimes the compressor reports that an image is already optimized, because re-encoding did not make it smaller. This is not an error; it means that re-encoding in the browser did not make the file smaller, so the original is kept. If you still need a smaller file, change something else: choose WebP or JPEG output, lower the maximum size or reduce the quality.

If the picture is a PNG graphic with few colors, a specialized palette tool may achieve more than a browser can. For most uses, however, converting to WebP is both simpler and effective.