This kind of depends on what image viewing (/editing) software you have on your computer. I’ll give an answer with microsoft paint for windows XP, since that’s a good candidate.
Open up paint and load the pic. In the image menu, choose the stretch/skew command, and then enter a small percentage number for horizontal stretch and vertical stretch… say 30 (Use the same number for each or the picture’s aspect ratio will get distorted.)
Take a look at the results, undo and enter a different number if it seems too large or small. Then either save on top of the original picture file or use ‘save as’ to save the altered pic under a new name.
Hope that helps.