Bear in mind that the "screen" is made up of two components: The LCD AND the "digitizer" which is the bit that makes it touch-sensitive.
It might only be the LCD that's borked but often the digitizer will be damaged too.
If you buy both you might find you could have saved a few quid if the existing digitizer is still okay or, OTOH, if you just buy the LCD and it turns out the digitizer is knackered you'll have to order one afterwards.
If it was me I'd just order both, together, and then, if it turns out the digitizer is okay, flog the spare one on eBay.
FWIW, you can buy iphone LCD/digitizers on eBay starting from £25-odd.
*EDIT*
Just saw the pic.
Given that the digitizer makes up the front panel of the phone, you're definitely gonna need both.
*EDIT2*
Also, how the hell did he do
that?

I've seen iPhones shot with a shotgun that looked better than that.