what program can i use?

This is the only one I've seen, but not for free...

http://www.fixerlabs.com/EN/upsize/sizefixer.htm

Suerte!

doing that without losing any detail is logically impossible, seeing that enlarging an image will make it blurred and it will lose its quality.
the only way to do it is ask the creator if he can remake it for a bigger resolution, else you can try any image editing tool to increase an image dimension (if the difference isn't that big then I guess it'll be fine - no much damage to the quality)

Try This Software,It's Free Too

image Resizer

try to change the format of the wallpaper

I prefer irfanview download it from download.com

there are plenty of programsthat will allow you to increas the size of an image file, the problem is it will be come pixalted - sorry

just use ctrl +e in paint
or if it doesnt work
try nero photoshop essentials it resizes images

adobe is the best one to increase the size of a wally..............