I have an image that is a solid color design on a transparent background. Now I want the same thing but with a white image so it can be transparent on a color background. Whenever I change the color to white (a small outline remains, but I dont care about that), then upload the graphic to a t shirt site, not only is the background transparent, so is the white area of the design!! I tried making it a faint gray, still transparent. I make it red or green etc, it shows up as expected. How can I make the white show up?

Hard to tell without the file to look at, but one thing to check might be the background color that is set in document properties does not always show up when exported. To have a background color show you have to create a large object/path and put it behind everything else.