Every photo you share online has probably been compressed. Image compression reduces the amount of data needed to store an image, making files smaller and faster to load. Understanding how it works helps you choose the right settings and avoid blurry or blocky images.
Why Compression Is Needed
A raw image stores color information for every single pixel. A 12-megapixel photo contains about 12 million pixels, and storing each one without compression would create a very large file. Compression finds smarter ways to describe the same picture with less data.
Lossless Compression
Lossless compression reduces file size without removing any information. When you open the file, you get back the exact original image. It works by finding patterns, such as large areas of the same color, and storing them more efficiently. PNG and GIF use lossless compression, and WebP has a lossless mode.
Advantages: perfect quality, ideal for graphics and text.
Disadvantages: files remain fairly large, especially for photos.
Lossy Compression
Lossy compression permanently removes some detail to achieve much smaller files. It takes advantage of how human vision works: we are more sensitive to brightness than to small color changes, and we rarely notice fine detail in busy areas. JPG is the best-known lossy format.
Advantages: very small files, great for photos.
Disadvantages: too much compression creates artifacts such as blocks, blurring, and halos around edges.
What the Quality Slider Means
When you export a JPG, you often see a quality setting from 0 to 100. Higher numbers keep more detail and create bigger files. For most web uses, a value between 70 and 85 gives a good balance. Above 90, the file size grows quickly while the visible improvement is small.
Resizing vs Compressing
These are two different things. Resizing changes the number of pixels, for example from 4000 pixels wide to 1200 pixels wide. Compressing changes how efficiently those pixels are stored. For websites, the best results come from doing both: resize to the size you actually display, then compress.
Common Compression Mistakes
- Re-saving the same JPG many times, which adds more quality loss each time.
- Using JPG for screenshots with text, which causes fuzzy edges.
- Uploading full-resolution camera photos to a website.
- Deleting the original after compressing.
Conclusion
Image compression is a balance between quality and size. Use lossless compression when every detail matters and lossy compression for photos where small losses are invisible. With sensible settings, you can cut file sizes dramatically while your images still look great.
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