Which file format should I use?
Web File Formats
When you're ready to add graphics to your Web pages, you'll immediately be confronted with a question: How should you format your images? Graphics in their natural state are too big to be quickly downloaded over the Web, so you have to compress them. The standard compression formats are GIF and JPEG (compression, by the way, doesn't shrink the dimensions of your graphics, it just reduces the size of the file). But is GIF or JPEG the better format to use? The resounding answer to this question is: It depends. It depends on the type of image you're working with, how small you want your image file to be, and the way you want your graphic to download. One of your primary considerations is the type of image you're working with. Photographs and graphics with lots of color fields, and particularly colors that blend and fade into one another, are best served by JPEG. For example, an unnerving picture of your coworker waiting to sing "Yankee Doodle" in a Club Med talent competition should be formatted in JPEG. If, on the other hand, your image has flat color fields, like the talent show poster, it will compress well in the GIF format. The reason for choosing JPEG for images with more complex color patterns is that this format enables you to save images with millions of colors, whereas the GIF option restricts you to 256 colors.
Another important issue is file size. JPEG permits a greater degree of compression than the GIF alternative, enabling quicker downloading times for larger graphics. And JPEGs appear to retain almost complete image quality for most photographs. But don't write off the GIF just yet. For one thing, the JPEG format does not work well for graphics that contain large fields of color - these color fields can break up and fragment and look terrible.
Furthermore, an image's compression format shapes the way a browser can download it, and browsers can do several things with GIFs that JPEGs don't support. One of the advantages to a GIF is that you can interlace it. Interlaced GIFs appear first with poor resolution and then improve in resolution until the entire image has arrived, allowing the viewer to get a quick idea of what the picture will look like while waiting for the rest. JPEGs can only arrive linearly, from the top row to the bottom row. Another plus is that the background of a GIF can be made transparent, so you see the background color of the browser window you're in.
Print Media File Types Which file type should I use?
Taken from Understanding File Formats by Bryan Chamberlain
You can spend a lot of time creating or altering your graphic files. When it comes time to save the image, it's important that you use the correct format. Otherwise, you'll lose image quality when you place the file in a desktop publishing document, or send it out for print.
Encapsulated PostScript (EPS, .eps)
The Encapsulated PostScript format is a vector file relying on the PostScript page description language to draw its image. The EPS is the preferred file type for high quality printing because when saved in CMYK mode, it easily breaks down for four color printing. This format can also contain raster information, even though it's not a raster format. EPS files generally contain a raster graphic as a screen preview--Mac EPS files use a PICT and PC EPS files use a TIFF graphic. EPS is the only format that supports transparent white in bitmap mode.
Versatility and compatibility make the TIFF image format the optimum choice for almost any project. It works on both the Mac and PC platforms, supports almost any picture bit depth, and allows various forms of compression. This flexibility also makes the TIFF format a Pandora's box. There are so many versions and types of compression for the TIFF file format that no current system can decode all of them. Furthermore, there's no way to tell how a TIFF will behave until you attempt to manipulate it. Try to use LZW compression or no compression at all for best results.
Avoid using PICT format in electronic publishing--PICT images are prone to corruption. Additionally, PageMaker always knocks out PICT images as color separations.
This is the native Photoshop file format created by Adobe. In this format, you can save multiple alpha channels and paths along with your primary image. You can't import this format directly into most desktop publishing applications.
The BMP file format is available in almost all Windows-based graphics applications, although it's primarily used in Windows application development. Do not use for print media.