When designing a graphic that may need to be resized it is always wise to create your images at a larger size than you think you will actually need. Raster images such as jpeg, gif, bmp, & png degrade in quality as the image is made larger subsequent to its design. Creating high resolution images ensures that you can shrink your image to a smaller size if needed without the noticeable loss in quality caused by pixelation.
An example of a situation when this would be necessary is in creating the logo for your small business that includes a photograph. You may only need the logo for envelopes and letterheads right now but if you design the logo that small you may have problems creating your billboard ad later. Professional graphic designers avoid this problem by using vector graphics that use mathematical calculations to render the same image at any size. When this is not possible they design raster graphics at a high resolution and simply shrink copies of the original graphic for specific uses.