Images used in a poster that are bitmap, are photos made of a fixed number of pixels or dots. Digital photos and scanned images are bitmap files, also called raster images. The images are made up of these pixels, or picture elements, that are so small and numerous it's hard to discern all the individual colors with the naked eye. Bitmap images are usually life-like representations. When the image has been applied to the poster and given a certain size, you can't re-size it without compromising the clarity of the image. If you try to enlarge the image, the pixels will just get larger and make the photo fuzzy and jagged. Adobe Photoshop and Corel Photo-Paint are programs that work with bitmap images. Bitmap data can be saved as EPS, PDF, GIF, JPEG, PICT, PSD and TIFF.
Images used on vector-based posters are made up of lines and shapes formed using mathematical equations. They are usually abstract images in logos or line images in print advertisements. The lines have starting points, direction and length. Rectangles have a starting point, height and width. Circles have a center and radius. Draw a square in a vector program, and go back and change its width and height without compromising clarity of the image. Bring it to the front or push it to the background, even after something else has been drawn on top of it. Each shape created is its own moveable image. Vector-based programs include Corel Draw, Freehand and Adobe Illustrator. Save vector images as EPS and PDF.
If you are using photos in your poster, the bitmap scheme is best because this is how photos are automatically represented. Details in photos are too complex to be vector-based. The bitmap representation is also best if you want to manipulate photos with blurs, interesting edges or filters. Bitmaps are not difficult to output, as long as your printer has sufficient memory. On the other hand, bitmap images can't be returned to and re-sized, and file size can be very large, because of the large amount of data being stored.
Vector-based images take up less disk space since the program is storing a simple mathematical equation. A bitmap image has to store the color information for all 10,000 or so pixels in a file. You can re-size vector images to be as large or as small as you want without losing quality, which makes them perfect for logos, maps and objects that need frequent re-sizing. However, if you size down vector images too small, thin lines in the image might disappear.