why do the icons look so stretched

I downloaded GIMP and am scaling the icons to 48x36 and I place them into the theme builder as an icon but it looks stretched out. so i went to a lower and it is horrible also. but the right measurement for the 8330 is such. What am I doing wrong. Can anyone tell me what I am doing wrong if they use GIMP would there be a step by step instruction on scaling them. that is all i am doing. is there an option i am missing. Thanks for everthing.

what i do is open the icon i want, create a new blank image at the correct size (48x36), then resize the original icon keeping the aspect ratio to the correct height (36), because the resolution isnt always going to be what you want if you only resize the height and keep the aspect, then copy and paste the new resized icon into the blank canvas and center it as you see fit, save.

try it out.