There are posiibly two issues here. If you resize one dimension without the other you will distort the image. Most programs will have a method to expand or contract a graphic while keeping the horizontal and vertical sizes in proportion. usually it envolves grabbing a corner instead of one of the sides and then dragging it. It may also require you to hold down either the shift, alt, or ctl key at the same time.
The second issue has to do with making images larger in general. expanding images will always lose detail and get pixilated the larger you make them. That is the reason you normally want to make graphics larger when you create them. You don't have the same issues when shrinking images (within reason).
