The two most important factors are resolution and color. When designing for DTG, you can use vector or raster files easily. I recommend raster because of the printer’s technology. Vector files are usually flat color and do not show off the strengths of the printer. Raster artwork (photos and high-end stock art) gives you the ability to print anything. Direct-to-substrate machines are color printers, so take advantage of that. Use their strengths because not all decorating technologies allow for this.
—Great Dane Graphics
