There’s no single answer to this question, but I can offer a few suggestions.
Professional textile designers frequently use Photoshop to create or prepare their designs. They might start with a scan of a painting or drawing or they might compose the image in Photoshop. The pros also sometimes use special plugins for Photoshop. But Spoonflower is not just for pros, so…
There are a number of free programs available for creating and manipulating images, a few of which were sent to me recently by Meggiecat, a terrific craft blogger:
Wintopo: Raster to vector converter newer, more robust than LineTracer
Note (6-23-2008): Do NOT try to upload TIF files created using Microsoft Image Composer. If that is the only program you have for preparing designs, save and upload a JPG instead of a TIF.
Find a high-resolution public domain image from one of
the amazing galleries available online. Be aware that you may only be able to employ an image of this sort for private, rather than commercial, use. The National Oceanic and Atmospheric Association, for example, offers incredible galleries online, including this one: Art of the NOAA Photo Library.
You could also print a photo or image of your own on fabric, although you should be aware that color shifting is likely to occur in a photographic image. The cleanest looking images printed on textiles have a relatively small palette of colors (under 30 or so), whereas if you examined photos and scans at the pixel-level of detail you would see thousands of colors.
Scan something — a kid’s drawing, a leaf, a swatch of vintage fabric. In order to have it print at its best you would probably need to clean up an image like this in Photoshop, but even without that level of attention you can get some rewarding results.
Tell me what you come up with and I’ll add it to the list!