Some issues to consider before you start are these summarized facts from Chapter 1:
Bitmap, or raster, images are resolution-dependent. A bitmap, or raster, image gets its detail from the number of pixels it contains, which is referred to as resolution and measured in pixels per inch (ppi). Photos are bitmap images, and paintings or illustrations that don't have well-defined shapes are usually bitmap images. Higher resolution usually translates into greater detail; lower resolution usually translates into less detail.
Ideally, pixels should be captured initially not added later. An adequate number of pixels needed to describe a bitmap image's detail should be captured initially, not added later (because where the heck would the new pixels come from?).
Vector images are resolution-independent. A vector image gets its detail from shapes and lines that are mathematically calculated and retains its smoothness no matter how much it is resized and regardless of how many pixels it contains. Solid-color logos and type are some examples of images that would ideally be created in vector format (but often are created in bitmap format).
Combination bitmap-and-vector images are resolution dependent. Although the vector image parts don't require a certain resolution, the bitmap image parts do.
Image resolution for bitmap and combination bitmap-and-vector images should usually be 300 ppi for print, 72 ppi for the Web. Old-timers who aren't nit-picky about terminology ...