Not sure which file format to order? EPS, TIF, and WMF files each have their advantages and disadvantages. I'll attempt to sort out, in very basic terms, the differences so that you can make an informed choice. Feel free to let me know of any errors or omissions or to offer more insight.

Just want gif? Gif files are included with the EPS, TIF, and WMF files.

EPS graphics.
EPS or vector graphics are resolution independant, meaning that you can enlarge them any number of times without having their resolution deteriorate. Vector images give you crisp, clean lines at any size. They don't, however, look as nice on screen as bitmap graphics do. If you have an illustration program like Adobe Illustrator, Freehand or Corel DRAW you can import and edit the images. Regardless of how they look on screen, EPS images print beautifully to a Postscript printer.

TIF graphics
TIF or raster graphics are bitmapped graphics. Raster images are best used for images such as photographs or paint style graphics. They are versatile and can be imported into most software but their biggest disadvantage is that they are resolution dependant. Bitmapped graphics can be reduced in size successfully but if you try to enlarge them you will be subject to serious deterioration as evidenced in the example at left. By the way Toonaday tifs are saved at a resolution of 300 DPI.

WMF graphics
WMF (Windows Metafile) is a Windows format used primarily for clipart. The majority of WMF files are vector based so they can be resized without degradation in quality. If you are primarily interested in using the images in applications like PowerPoint then WMF is your best choice. In the PowerPoint slide on the left, notice that the EPS graphic looks terrible. Not only is it jaggy but it has an ugly white box around it making it unsuitable for placing on colored backgrounds. A tif file will look smoother but will have the same white box around it. The gif file is better in that there is no white box around it but you have the same problem as with trying to place a gif on a colored web page; that white fringe around the image. That can be overcome in an image editing application but it means more prep work. The WMF image by contrast, looks great on any background and can be sized up or down with no degradation.

The Bottom Line.
If you have a postscript printer or will be creating documents that will be output to a postscript device then your best bet is to go with EPS graphics. Better yet, if you have a program like Photoshop that allows you to convert to other formats then EPS is definitely the way to go. You'll have the best of both worlds. If, on the other hand, you don't print to a Postscript device and your software doesn't support EPS graphics (although most do) then maybe TIF files are the way to go.
And finally if you want images primarily for PowerPoint then go with WMF.

For more detailed information on these and other file formats go to:
http://graphicssoft.about.com/cs/formatsnonweb/