The odds are against you when designing images for the web. There are so many variables that cause a graphic to display differently than intended. Resolution, bit depth, platforms, browsers, palettes, and file format all play a major role in how the images you create are viewed. An image size is determined by the height and width in pixels. The monitor setting then contributes to the size the image appears on the screen.
Resolution is the number of pixels displayed per inch.
Your web images should be saved at 72 ppi. The newest models can display millions of colors. Results are not always pleasant since the quality of the image is lost. Monitors may be set at different levels of display. Each has it's own palette, that is, the number of colors they relate to.