A bitmap is a map of bits (pieces of computer information) describing all the points in a rectangular grid of dots ("picture elements" or "pixels".) Photographic images are usually in bitmap form. Bitmaps are also refered to as raster images.
Dimensions:
Bitmapped images are rectangular grids but just because you know the dimensions of a grid in pixels, you don't necessarily know how big (in linear measurment like inches) it is, unless you know how close together the samples, or pixels, are. That is were resolution comes in.
Resolution
The term resolution is used in so many different contexts that its easy to get confused.
When you scan a picture that is 3 inches on each side at 100 spi, you know that the bitmapped image has 300 samples on each side (100) per inch.
Bit Depth:
A bit is the smallest piece of computer information- it's either a 0 (white) or a 1 (black). Therefore, a one bit image is made up of only 2 values, black and white (with no grays). 8 bits gives you 256 levels of gray (or color). An 8 bit image is 8 times the file size of 1 bit: it uses 8 bits to describe each dot instead of one. Digital imaging primarily uses the three primary colors; Red, Blue and Green (RGB), to make all other colors. An 8bit RGB image has 16 million possible colors in it, that is: 256x256x256. See this page for further explanation.
Resampling
You can resize a bitmapped image. It is usually best to only downsample or discard pixel information. In general, try to avoid taking a file that does not have enough resolution or print size and changing it with an image editing program such as Photoshop.
In Photoshop's Image/ Size/ dialog box, you can upsample through interpolation. This means that the computer is guessing based on the existing pixels in the image so that you can have either a higher resolution, a larger print, or both. Often this produces an image that is less that optimum in sharpness and smooth color.
Increasing the resolution of a bitmapped image increases file size geometrically, so if you double an image's resolution, you increase file size by a factor of 4.
rdh for: Peggy Jones, MiraCosta College