In order to properly choose which T-shirt printing method is right for your design, you must consider the design itself. Simple, silhouette and curvy style designs work best with "Plot Printing". Complicated detailed designs that resemble photos should probably use digital. Ones that fall in between should use screen printing. Regardless, simple designs that can be interpreted from a good distance are the best way to go.
Screen printing - This is the process of transferring an image or text to a garment using a stencil in which a design is imposed on a screen of silk or other fine mesh like polyester. The ink is then forced through the mesh onto the printing surface. This is not a print-on-demand type and is fairly expensive. Think of comic book art and how they pencil and then ink in the shadows to add definition. That is the type image that works best using this method.
Digital printing - Is the fastest way. Designs created with Adobe Photoshop or pictures taken from a digital camera are the easiest ways to produce designs. The perks are that there is no minimal order as it is print-on-demand which is cost effective. White areas are left blank and show up as the color of the fabric. When printing on dark fabrics a thin layer of white is applied to the garment first then dried before printing over it with the design.
Just about anything can be printed if you take into consideration the bleed factor which could result in blurry and smudged images. PNG format at 300 dpi is the ideal resolution for this process. Take care to make the design full sized to begin with as any increase changes in the size distort and pixelate the design elements. Check the "Gamut Warning" before saving.
Plot Printing - Consists of both Flock and Flex Print, This process transfers a simple silhouette style vector image from a special foil through an immense amount of pressure and heat. Flex images are smooth, a little plastic like and a tad bit glossy. Flock images have a fuzzy velvet like texture and appear slightly more elevated. Think of drawing just the shadows and how that would appear without color. 3 colors plus the color of the garment itself is used to print.
Stencils are cut-outs that divide sections by color in which to be filled.
Digital designs are made up of pixels. Adobe Photoshop, GIMP and Jasc Paint Shop Pro work well for designing software. Keep in mind that paper and fabric have different bleeding and absorption. JPG, GIF, PSD, and PNG are the acceptable file extensions.
Vectors are made with curves as opposed to pixels meaning they scale. Vector graphics can be made using software like Adobe Illustrator and Corel Draw and usually have an AI or EPS file extension.