Contrary to what many programs tell you, not all image files are interchangeable. For faster downloads, image files need to be compressed. Each image file compresses the information in a different way.
GIF : Graphics Interchange Format
(the "G" is hard, like GIFT, not soft like JIFF)
GIFs are bitmap images, which save space by limiting the # of colours - (LZW) lossless compression.
. This limited colour pallet is great for small images with few colours (arrows, bullet points, and other line based art). GIFs also have the unusual ability to store multiple frames, which when opened play back each frame in sequence, making an animated GIF. GIFs can also allocate one of the colours to be transparent. Full colour photos are much richer with millions of colours, instead of 100s, so GIF is not your choice for photographs or for printing.
JPG : (from Joint Photographic Experts Group)
To achieve those rich, full colour photos, that don't take up much space, JPG is your number one choice. JPG allow you to compress your photos, usually at 10:1 compression - lossy-compression - allowing you to fit a very high resolution photo in a small file. Most digital cameras shoot in JPG to save space, for the maximum amount of photos on your digital media card. Images can range from small web graphics to huge poster sized images.
PNG : (Portable Network Graphic)
PNG is an interesting mix between JPG and GIF. A PNG is a bitmapped image, like GIFs, but they use lossless compression to save space. PNGs have the limted colour pallet like a GIF, but their colour pallet can be Millions of colours, instead of the 100s found in GIFs. PNGs can have the beautiful full colour photographic capabilities, and impressive compression, of a JPG - with the added bonus of having a full range of colours that can be used for transparency (opposed to the single color of a GIF.) PNGs, however, are another web based file type, and are not the best for printing.
TIF
A TIF file is generally a print ready bitmapped image file. It can be saved at very high resolutions and can be RGB (for the web) or CMYK (for printers).
Not everyone will have the software to open a TIF file so it is best to save as a nother format if you are just sharing holiday snaps.
EPS
An EPS file is a vector file - instead of small dots, the image is a series of mathmatical formulas that create lines and colours that make up a lineart image. EPS is a fantastic formula for printing because, unlike a bitmapped image, a vector image can be shrunk down or blown up to any size without loosing quality. It is NOT for photographic images, and can also tricky to open without the correct software.
If you have any questions, feel free to ask our resident graphics guy, DavidB@yourplanb.com