Pick the final surface color
Use the color of the page, document, or marketplace where the JPEG will appear. White is common, but it is not universally correct.
Inspect semi-transparent edges
Shadows and anti-aliased logos contain partially transparent pixels. Compositing against the wrong matte can leave a pale or dark halo.
Do not confuse format with compression
Changing PNG to JPEG may reduce a photograph dramatically, but flat illustrations and text can become less crisp.
Keep a transparent master
Export the delivery JPEG as a derivative. Preserve the original PNG when the artwork may later appear on a different background.
Check the matte at actual size
Place the converted JPEG on the real destination color and inspect curved edges, small lettering, soft shadows, and glow effects at 100 percent zoom. These pixels often mix foreground color with transparency, so a matte that looked harmless in a thumbnail can create a visible fringe. If a halo appears, return to the transparent source and choose the destination background before conversion; changing the JPEG afterward cannot reconstruct the discarded alpha values. Avoid using a screenshot of the transparent image, because that bakes in an uncontrolled browser or editor background.
Separate color change from quality change
JPEG export performs two decisions at once: transparent pixels receive the matte and the resulting image is encoded lossily. Begin with a high quality value so you can judge the background edge without heavy compression noise. After the matte looks correct, lower quality only if the receiving system has a size constraint, and compare each result with the untouched PNG. Repeatedly reopening and saving the JPEG compounds artifacts. Always produce each trial directly from the original transparent master.