Raster images are made up of pixels, dots of color that work together to form the image. Photographs are always raster files, as well as most images you see online. Raster images will gradually lose quality as you scale them up, creating a rough, pixelated appearance.
Vector images are not comprised of pixels. Instead, they use math equations to determine the appearance of the image. Because of this, vector images can be scaled up or down to any size and they will never decrease in image quality, which makes them ideal files to send to be printed in large formats.
Every raster image is made up of pixels; single dots of color that, together, form the image. As an example, a 2-megapixel image has 1,920,000 pixels (1200x1600).
.jpg / .jpeg
A JPEG (saved with the extension .jpg) is the most common file type for an image. Most digital cameras take a .jpg image by default. This is a raster file format that uses color compression to reduce the size of the image file. The compression amount can be adjusted, but .jpg files typically achieve a 10:1 reduction in file size with very little noticeable loss in image quality.
.tif / .tiff
A TIFF file (saved with the extension .tif) stands for Tagged Image File Format standard. This is a raster file that can be saved uncompressed without any degradation of the image.
An EPS file retains the information of a vector file. This means that .eps images can be scaled to any size without loss of image quality. This is a great file type to send to be printed, especially for large format prints.
Color profiles are generated for printer and paper combinations so that accurate reproduction of colors is possible. They are also very useful in obtaining color matches between monitors and printers to ensure that what is seen on the screen is an accurate representation of the final printed image.
RGB stands for the Red-Green-Blue color profile. This is typically used in monitors and digital cameras. Colors are defined by the levels of red, green, and blue measured on a scale from 0-256: therefore, pure black is 0-0-0 and pure white is 256-256-256.
CMYK refers to the Cyan-Magenta-Yellow-Black color profile, usually associated with four-color printing. Each color is measured on a scale from 0-100. CMYK is a subtractive color scale, so pure black is 0-0-0-100 and pure white 0-0-0-0.
DPI stands for dots per inch, and refers to the number of pixels within one square inch of an image. A higher dpi results in a higher quality image. The dpi that an image needs to be depends on the size of the print and the distance it will be viewed from. In offset printing, such as magazines or newspapers, 300 dpi is the standard. In large format printing, the dpi can be much lower.
Resolution expresses the image quality, and is defined by pixel density (dots per inch) or image size (pixels tall x pixels wide).
Raster images are often resampled when being enlarged in order to lessen the noticeability of pixelation. Resampling can “soften” images, particularly at sharp borders between colors, but when done by a knowledgeable professional it can enlarge an image with very little loss of image quality.