Hi Mella
Check this site out:
http://www.myimager.com/
It lets you upload a photo, and then resize (and lots of other funky things) online - then you just save it back to your computer

When you've resized the image, at the bottom left it will tell you the size in pixels, which should make it easier to see if it can be used here
Let me know if you need any help!
Mark