Start with the destination
Before optimizing an image, decide where it will be used. A photo displayed at roughly 1200 pixels wide on a website usually does not benefit from remaining 4000 pixels wide. A logo that needs transparent edges has a different requirement from a photograph placed on a white background. The correct operation follows from that destination.
1. Pixel dimensions control how much image data exists
Width and height determine the pixel grid. Cutting a 2400 × 1600 image to 1200 × 800 keeps the same 3:2 aspect ratio while reducing the pixel count from 3.84 million to 0.96 million. That often matters more than adjusting an encoder slider. UseResize Image when the source is materially larger than the size you need to display or deliver.
Upscaling is different. Increasing dimensions does not recreate detail that was absent in the source; it asks the browser to interpolate more pixels. Upscale only when a downstream system requires larger dimensions and verify the visual result at the intended size.
2. File format controls capabilities and compression behavior
PNG is useful for lossless graphics and transparency. JPEG is widely supported for photographs and uses lossy compression, but it has no alpha transparency. WebP can represent photographic content efficiently and can preserve transparency in browser-generated output. A format change is useful when the destination accepts the new format and its properties fit the image.
Use PNG to JPG when a transparent background is not needed and JPEG is appropriate for the destination. UsePNG to WebP when WebP is accepted and you want to keep a transparent-capable format.
3. Encoder quality trades detail for file size
A quality control affects lossy encoding; it is not a universal percentage of visual quality. Two images encoded at the same setting can produce very different file sizes. Fine texture, noise, gradients, and sharp edges all change how well an image compresses.
Image Compressor is useful when the dimensions are already appropriate and you want to compare a smaller JPEG or WebP output. Start near a moderate-high setting, inspect the result at normal viewing size, and lower it only as far as the destination permits.
A practical sequence
- Resize first if the source dimensions are much larger than the destination.
- Choose a format that supports the required transparency and compatibility.
- Adjust lossy quality while comparing both appearance and byte size.
- Keep the original file when the optimized output will be reused or edited later.
An already optimized image can become larger after conversion. That is not automatically a tool failure: different encoders and formats have different overhead and compression strengths. Compare the actual output instead of assuming every conversion must reduce size.