How to delete photo from iphone 3gs?

I have ended up with 1000+ pictures on my iphone 3gs, how do I get rid of them? Thank you

Added (1). Hi Clare,
Thank you, thats how it works on my itouch but the trash can doesn't appear on the 3gs. Only with the pictures taken with the iphone. Not the 1000+ it got on when I sync'd with my laptop! Frustrating!

Go to the photo app, select the picture you want to delete… It should blow up to full screen. In the bottom right corner, there'll be a small white trash can. Hit that.