Note: JPEG images are smaller due to more efficient compression, and (usually) higher quality, since they are 24 bit color. However, if you have an 8 bit display, they may take longer to display, since they must be quantized to the available color space. Usually, the time to quantize is small compared to the download time.

JPEG uses `lossy compression', which means that images will degrade if decompressed and recompressed repeatedly. However, the information lost is generally invisible to the human eye, unless the image undergoes compression more than, say, 5 times.

PNG images, on the other hand, are quantized to a small number of colors, (<= 256) which is a much more lossy process than JPEG compression, but it only happens once. This also makes it difficult to view more than one PNG at a time on an 8 bit display and have all of them look good, unless they are all quantized to the same color space. Because of the ways that the two formats degrade images, it is usually a bad idea to convert PNGs to JPEGs, although the reverse is generally acceptable.

There are various other image format considerations, and those interested should read the
JPEG FAQ.