I have a problem. The submitting guidelines say a picture has to be a maximum size of 15 x 19 inches and be 640 x 800 pixels. Well anytime I try to make it that size of pixels and get much less then 15 x 19inches. And anytime I make it 15 x 19 inches the pixels are way more then 640 x 800. How can I get my picture to do both so I can submit it?
640 x 800 Pixels is the Submission Preview only
Your original artwork should be much larger than that, otherwise your design will look dreadful when scaled up to T-shirt print size. The original artwork is what TeeFury will ask you for if they accept a design.