How to compress a PNG and keep its transparent background

Check whether the original is transparent

A logo may appear transparent on a white page. Put it on a dark background or open it in an editor with a checkerboard. A white rectangle may be part of the image, and a pale fringe may appear only against dark colors. PNG supports transparency, but it can also contain a fully opaque image. Compression will not remove a painted background.

Choose an output format

• PNG: Use it when the destination requires PNG or sharp text edges matter. Reduce oversized dimensions first.
• WebP: Try it when the destination supports it and compare the result with PNG.
• JPG: JPEG cannot store transparency. Astrotor places transparent areas on white in its JPEG output.

Test a logo at its actual display size

Suppose a 2,400-pixel logo is displayed at 200 CSS pixels. Export a 400-pixel copy as a starting point for a display with a pixel ratio of two, then compare it with a 200-pixel copy. Check the actual page rather than judging only at 100% zoom. Inspect shadows, small lettering and antialiased edges on two backgrounds. Keep the original vector file for larger uses.

Find where the white background appeared

If the downloaded image is transparent on your computer but white on the site, inspect the file the page actually serves. The uploader may have converted it to JPEG. If the served file is still transparent, inspect the page background. Comparing the source copy with the delivered copy shows which step introduced the problem.