Frequently Asked Questions
The reason this should concern you it to do with compression (how large the image file is): the more compression the smaller the file and the faster it will download. Photographic images require different compression techniques to non-photographic images.
Photographs should be in JPEG format (extension .jpg or .jpeg). JPEG is a lossy compression format, so you should retain the original image in case you need to manipulate it later. JPEG conversion always has a "twiddle factor" called 'compression,' or 'quality' or something like that. The greater the compression (or the lower the quality) the more detail is discarded (which is why it's called a lossy compression format). Reasonable settings of compression/quality (often your image manipulation program has a default value which is reasonable for most images) will make the resultant file very much smaller without any visible change to the image (unless you compare the two very, very closely). JPEG is all about discarding detail we cannot see anyway.
This type of image has often has sharp transitions of colours rather than smooth blending. Icons such as the one to the right (which is the FAQ "folder" icon for this knowledgebase) are typical non-photographic images. There are gradients of colours, but not as subtle as in a photographic image. The formats discussed here are lossless (every detail of the original is preserved).
The two formats commonly used for this type of image are GIF and PNG.
GIFs used to be the most common format for non-photographic images. In 1994 it was realized by Unisys that GIFs utilized technology they had patented and they decided to try to enforce that patent (which expired in 2004 so GIFs may now be created/used freely). However, Unisys' actions outraged some netizens and PNG format was born.
PNG was conceived during the 10-year patent battle over GIFs. Technically it is superior to GIFs in every way. The resultant files are almost always smaller than GIFs (when they are not the difference is negligible). Interlaced PNGs (a way of displaying a crude image quickly then refining it as more data is downloaded) are better than interlaced GIFs. PNG not only has the ability to specify a transparent colour (as do GIFs) but it can specify an alpha channel (translucency) for each pixel (which GIFs cannot).
The downside of PNG is that Microsoft's Internet Exploder was late to support PNG. It was later still to support transparency. And it was even later still to support the alpha channel.
If you need backward compatibility (with no alpha channel) go for GIF. If you want a good format (at the exclusion of older versions of IE) go for PNG.