It should be the exact same name as it shows on your springboard. I couldn't even find this "where to" app in the appstore (it is an appstore app right?) but yeah, if it's called where to then it should be "Where To.png" (try "Where_To.png").
edit: Searched google and it looks like the developer "tap tap tap" took "Where To?" off the appstore for some reason (which is also odd because apparently it was a best selling app), don't see why this would effect icon editing tho...
edit2(I'm hooked on this story): they're selling the app, check their blog post here:
http://www.taptaptap.com/blog/the-ea...-a-proven-app/