It’s important to know that in digital graphic design, there are the two types of digital artwork used in creating any document - vector artwork and bitmap images.
Vector-based artwork is created in vector-based software such as Adobe Illustrator. Vector artwork contains vectors or points that have been “plotted” and connected to form objects. These easily manipulated objects can be lines, boxes, circles, curves, polygons, text, etc. and can be given a variety of attributes such as colors, gradients, sizes, line weights, etc. Vector graphics can be enlarged to any size without a decrease in print quality.
Bitmap-based image is captured with a digital camera, scanned or created in bitmap-based software such as Adobe Photoshop. A bitmap image is simply a collection of squares called pixels, laid out on a big grid. Any enlargement to a bitmap image may cause the image to appear jagged or “pixelated.”
Any artwork such as line art, illustrations, text, graphs, charts, etc. should be created as vector artwork where resolution will not be a factor. This will prevent any headaches in the event the file will need to be edited or enlarged.
If it’s a photograph you have no choice, they are all bitmap images. When working with bitmap images keep in mind the actual size, resolution and how much of an enlargement is needed if necessary.