Not sure on
DTG as i've never used one or had anything printed with one but regarding the image you will probably need a good quality photo.
It's always best to have the best possible quality image to start with. I've done separations for silkscreening from a digital photo of a mural in the past. We used the highest resolution setting on the camera, good lighting and a tripod for best results. Even with this great picture i still needed to play with it in photoshop, adjusting contrast, saturation and levels and a bit of cleaning.
It's possible to use a digital image but for ideal results you will still need to manipulate the image before going to print.