Are these in your Camera Roll or in the Photos app?
If in the Camera Roll you can delete them by opening the Cameral Roll to display thumbnails, ap the "move" icon in the center of the screen at the bottom, then tap the photos you want to delete, which will put a check mark on them. Then tap the Delete button.
If in the Photos app connect your phone to iTunes on your computer, click on its name, then click the Photos tab and select the ones you want to remain on the phone, and uncheck the others. Then sync and the unchecked photos will be removed from the phone, but will remain in iPhoto.
Photos and albums that were copied to your phone by syncing with your computer are deleted by doing another sync. But before clicking on Sync deselect the unwanted albums in itunes on your computer. Then click on Apply or Sync. The albums will be deleted from your phone but they will remain in iPhoto on your computer
I totally hate the way Apple deals with the photos on the iPhone. I was facing the same issue and found this simple solution on another board:
Use Import in Preview
After you connect your iPhone to your Mac, you are able to import photo. You just need to use import in Preview.
File…. Import from iPhone 4s….
Use Preview to delete photos
You can see your photos thumbnails in the Preview Import screen. Choose any files that you want to delete, select those files and click on the DELETE button. Make sure you have backup all your important photos before delete, because this will delete those photo permanently. Use this with careful.
If you want to delete all photos from the iPhone, you can select all photos by using Command + A, then only click the delete button.