Sometimes you may find the work is clear when you design, but it may appear blurry after you download it. When you resize your work, the pixels of your image can shrink or enlarge which will easily lose the quality. JPG is a kind of lossy compression format, compressing the larger file into the smaller one. If you save the work as JPG, you may get a poor quality work.
To get a high quality work, you can try to save it as PNG format. PNG format can compress the work without losing the quality.