Helen,
This may sound goofy, but make sure the ORIGINAL image is sharp,before you go any further. If the original isn't sharp, then everything you do after that will add to the problem down the road. Also, don't sharpen your images for upload,(you can slightly sharpen them, if you have to),since the printer will sharpen the image, "to size". Example: if you sell a large print of one of your images, 26x40, the printer will now sharpen the image, to look good as that 26x40 and YOU can't assume that ALL your images will be sold large, so sharpening them for a large size/sale and then selling one as a greeting card, the image will be way over-sharpened. Does this make sense?
Rich