What are Raster Images (tif, jpeg, gif, png)?

All graphics fall into two main catorgories, Vector or Raster.

Raster images are those made up of pixels, such as photographs or files created using Photoshop (tifs, jpegs, gifs). The advantage of a raster graphic is a photo-realistic quality. No other file type can match a raster graphic for realism. The disadvantages are numerous. One disadvantage is file size. These files have a larger file size as compared to vector images, sometimes significantly so.

Another disadvantage is a resolution/scaleability issue. If you could stretch your image three times its original size you end up with pixels that are three times bigger, making your image look pixeled or blurry. Because of this, a raster image that looks OK when small will look very rough and jagged when it is blown up. This is why using photographs from a website doesn't work well for print. Blowing up the web graphics to the higher resolution just results in very blurry artwork.

A final disadvantage of a pixel based graphic is color separations. These graphics separate fine as cyan, magenta, yellow and black (CMYK), but if you need your graphic to separate into Pantone 354 (green ink) and Pantone 273 (purple ink), it’s almost impossible to get good separations. For creating images that are to output as spot separations, vector graphics should be used.