Image type matters
Use PNG for screenshots, interface assets, diagrams, text and transparency when exact pixels or crisp edges matter. Its lossless coding avoids additional image artifacts, though the file may be large.
Use JPEG for photographs where smaller files are valuable and some loss of detail is acceptable. JPEG has no transparency, and repeated lossy saves can compound visible artifacts.
Conversion does not change the source content
Saving a PNG as JPEG does not create photographic detail; it discards pixel precision and composites transparency against a background. Saving a JPEG as PNG does not restore details already lost and usually increases file size.
Keep a source master and export a delivery copy for the intended use. If a modern web format is appropriate, WebP can offer both alpha support and lossy compression, but compare the result first.
Frequently asked questions
Should photographs be PNG?
Usually not for web delivery; JPEG or WebP often stores photos more efficiently, unless exact pixels are required.