I tend to shoot a lot of pictures with my iPhone and have accumulated a large number (more than 4,500) on it. I sync them with iPhoto regularly but have tended to not delete them afterwards. There is no built-in way to perform bulk deletes on the iPhone but I recently ran across a tip at gigaom.com which clued me in to using the built-in Mac app called Image Capture to bulk delete. You should find Image Preview in the Applications directory (run the Finder app, click on Applications, scroll until you see Image Capture and run that. If your iPhone is attached to the Mac you should see it in the Devices list on the left. From there you should be able to view the pictures on the iPhone. You can select the ones that you want to delete and then click on the little red circle icon with a slash through it on the bottom left (no tool tip popup oddly enough). The delete processing may take a while so be prepare to wait but it appears to do the trick.