How to delete pictures from my iPhone?

Without doing it picture by picture? The pictures I've taken on my phone are taking up a lot of memory. I've already copied them to my computer, so how do I delete them from the phone now? The only way I can see is by hitting the trash button while viewing it on the phone, but I don't want to do that for 1200 photos, since it takes a few seconds to load each time. Is there a way I can delete them all at once?

Just press delete or select all then delete

Photos>album>top right small button with an arrow>choose photos>press delete!

From Home Screen:
- Tap Photos
- Tap Desired Album
- Tap Arrow in top right corner
- Tap all the pictures you wish to delete
- Tap the delete button on the botton right corner

You can also choose to delete the photos off of your phone after uploading them to the computer usually as well (I know for sure you can with iPhoto on a Mac)