High-resolution photos and screenshots are often several megabytes, leading to slow uploads, email attachment limits, and storage consumption. Compression can significantly reduce file size, but many worry about quality degradation. In fact, with the right method, the difference in compressed images is often imperceptible to the human eye. This article will help you understand the compression principles and provide practical advice.
Why Images Can Be Compressed
Image compression is divided into two types: "lossless" and "lossy". Lossless compression re-encodes the image without losing any pixel information, resulting in limited size reduction. Lossy compression discards details less sensitive to the human eye to achieve a much smaller file size; both JPG and WebP are lossy formats.
For photos, moderate lossy compression can reduce file size by more than half without significantly affecting the visual perception. This is the key to "not losing perceptible image quality".
Choosing the Right Quality Setting
Compression quality is typically represented by a value between 0 and 100. Empirically, setting the quality between 70% and 85% is the sweet spot for balancing file size and clarity: the file size decreases noticeably, while quality loss is hard to detect.
Below 60%, photos may show visible color blocks and blurriness; above 90%, the file size reduction is minimal. It's recommended to start at 80% and then fine-tune based on the preview.
Choosing the Right Output Format
WebP generally offers smaller file sizes than JPG at the same quality, making it ideal for web images. JPG has the best compatibility and is suitable for photos. PNG is a lossless format; adjusting quality is ineffective. Compressing PNG mainly involves reducing its dimensions or converting it to JPG / WebP.
Limiting Dimensions is Often More Effective Than Reducing Quality
If the image's actual display size is much smaller than its original resolution (e.g., a phone's original photo used as a web thumbnail), limiting the longest edge to an appropriate pixel value can drastically reduce file size. Since it's scaled proportionally, clarity remains more controllable.
Compress Safely with Online Tools
Use the image compression tool from qktools. You can drag multiple images at once, freely adjust the quality and longest edge size, choose the output format, and download the compressed images individually or as a package. All processing happens locally in the browser; images are not uploaded, ensuring privacy.
If you also need to convert formats or crop dimensions, you can use the format conversion and crop/resize tools together. The entire image processing workflow can be completed offline.