JPEG is widely supported and always opaque. A PNG with transparency therefore needs an explicit matte before JPEG export. WebP can retain the transparent canvas and often produces smaller photographic files, but the receiving system must support it. Test the real destination rather than assuming that the smallest Blob is automatically the best choice.
Choose the JPEG matte before encoding
Transparent and semi-transparent pixels are composited against the chosen opaque colour before JPEG encoding. White is a common default, but it can create visible halos when the image will sit on a dark page. Match the expected destination background and inspect edge pixels in the generated preview.
Know what re-encoding changes
ImageCompress preserves decoded pixel dimensions, not the original encoding. It does not promise metadata, ICC profiles, DPI, animation, byte identity, losslessness or professional colour fidelity. JPEG replaces transparency with the selected matte. Use ImageResize separately when changing dimensions is acceptable and a smaller byte budget cannot be reached by quality alone.
Inspect the actual output
The preview and download point to the same browser-created Blob, so compare edges, gradients, text and transparent areas before using it. The byte count is exact for that Blob, but another browser or codec version may produce different bytes from the same visible settings. Keep the original file until the destination accepts the result.