Smaller image files can reduce transfer time, but a percentage saving on one file is not the same as a percentage improvement in page speed. Start with the image’s purpose: a product photo must show useful detail, while a screenshot must keep its labels readable. Keep the original so each experiment begins with the same source.
1. Choose useful pixel dimensions
Suppose a photo is 4,000 × 3,000 pixels but appears in a 600-pixel-wide card. A 1,200 × 900 candidate provides two image pixels per CSS pixel at that card size. That is an example to test, not a universal requirement: a full-screen zoom view may need more detail. Use the Image Resizer with aspect ratio locked, then inspect the result at the size visitors will see.
Reducing both width and height by half leaves one quarter as many pixels. Encoded file size will not necessarily fall by the same ratio because format, image detail and encoder settings also matter.
2. Compare formats using the same source
Try JPEG or WebP for a photograph. For a screenshot or logo, compare PNG with a suitable WebP export and inspect text and transparent edges. JPEG cannot retain transparency; the compressor fills transparent areas white when exporting JPEG. The format comparison guide explains these trade-offs.
3. Lower quality in small steps
In the Image Compressor, start with a moderate JPEG/WebP quality setting, then compare the original and result. Check hair, gradients, small lettering, and high-contrast edges. Keep the smallest version that remains useful for its intended display. Repeatedly recompressing the previous output can accumulate loss; return to the original for each test.
The quality slider does not control PNG output in this browser canvas workflow. Target-size mode estimates a nearby size for JPEG/WebP and may exceed the requested KB limit. If a form imposes a hard limit, check the downloaded byte count and resize the dimensions if necessary. The tool’s KB calculation uses 1,024 bytes. See MDN’s canvas encoding documentation for the format and quality behavior.
4. Record the result, not an assumed saving
Use a simple comparison sheet: original bytes, output bytes, dimensions, format, quality setting and a pass/fail visual check. For example, a hypothetical file reduced from 800 KB to 200 KB saves 75% of its bytes: (800 − 200) ÷ 800 × 100. That arithmetic does not predict a 75% faster website.
5. Test delivery on the page
Upload the chosen output and confirm the page actually requests it. Developers can use srcset and sizes to offer several widths, and explicit width/height attributes to reserve layout space. See Google’s responsive-image guidance. Check the browser Network panel for downloaded bytes on mobile and desktop.
Do not lazy-load an image that is the page’s Largest Contentful Paint element. Images farther down the page can be considered separately. Compare repeated lab runs and, when enough real traffic exists, field data; Google’s LCP guide explains why resource discovery and rendering matter alongside file size. A single compression result does not establish a ranking or Core Web Vitals improvement.