Graphic images on your computer come in two different forms, raster images and vector images. Raster images are made by programs such as Photoshop and Corel Photopaint. Vector images are produced by Adobe Illustrator, Freehand, and CorelDraw. While the results from these different programs can look similar, the ways in which they are made are very different.
Raster images are made up of millions of individual squares, or pixels, of various colors. The more pixels you have, the better the image will look. The number of pixels, or resolution, is usually expressed in dots per inch (dpi). Images on the Web are shown at 72 dpi. A high-resolution image would run 300 dpi or higher. Some types of printing can get into the 1600 dpi range. Much like a mosaic, a raster image can look smooth from a distance but as you zoom in closer you can see the individual pixels.
A vector image is drawn from mathematical formulas for lines and curves and is redrawn each time you zoom in for a closer look. The quality of the image stays the same regardless of the level of magnification. Vector files, since they are formulas instead of information on millions of individual pixels, tend to use less memory than raster files.
The edge of a circle, rendered as a raster image, may look smooth initially, but eventually as you zoom in you will see a jagged stairstep edge of the individual square pixels that make up the image. A lower resolution image will look jagged or pixellated much sooner than a high resolution image. A circle's edge in vector form will always be smooth no matter how close you zoom in since it is recalculated each time you change the view.
Raster based programs are best at working with photo-realistic images and make subtle (or bold) changes in color, shadow and texture. Vector based programs excel at easy control of edges and tend to produce a more graphic style of art. Regardless of the type of image, higher resolution is always better for producing a good result. While your image may be vector or raster, or even a combination of the two, the Art Staff at ExpertShirt.com will work hard to translate it into the best textile printing possible.