Rosemary,
I assume that when you get ready to create your art, you establish a "canvas" size. Maybe you can create a much larger canvas when you begin. Maybe 6000x6000 or so. And then as mentioned, save as quality 12 and you should be fine.
I would also raise your prices, as they seem low to me,based on the quality on your site, $22 to $27, at least. Find other artists that you feel you are comparable to and look at their pricing structure and then "borrow" that!
And finally, try and increase your "keywords", so you have a better chance of showing up in any searches.
Welcome!
Rich