How to Choose Thumbnail Dimensions
A thumbnail should match its display aspect ratio closely to avoid unnecessary cropping or stretching.
The practical idea
A thumbnail should match its display aspect ratio closely to avoid unnecessary cropping or stretching. The best setting depends on where the image will be used, the amount of detail it contains, and whether transparency or very small file size is important.
Start with the destination
Choose dimensions and format based on the final destination rather than repeatedly converting the same file. Keeping an untouched high-quality original makes it easier to produce new versions later.
Check the result visually
File size and dimensions are measurable, but visible quality is still important. Compare the processed image at the size where people will actually see it.
Avoid unnecessary processing
Repeated lossy compression can accumulate visible artifacts. Whenever possible, create each delivery version from a clean original rather than from an already compressed derivative.