Sometimes you just need to wipe everything out and start fresh. Whether it's because you're running out of storage space on your iPhone, you have more embarassing selfies than you can handle, or some other reason, there are times when you might want to delete all the photos you have stored on your phone. Fortunately, there's a pretty easy way to do it.
If you have a Mac
The easiest way to delete all your iPhone photos at once involves hooking it up to a Mac. If you have one, great! Just connect your phone using the appropriate cable and open up Image Capture (which should be located in your Applications folder). A window should appear showing all the photos on your phone. Click on the window, then press Command + A to select all your pics. Next, click the delete button (it looks like a circle with a line through it), press Delete when the warning prompt appears, and that's it - you'll have removed all of the photos from your iPhone.
Using only your iOS device
If you don't have access to a Mac...well, unfortunately, there's no convenient way to erase all photos at once using only your iOS device (unless you want to go full nuclear and wipe everything on your phone in Settings > General > Reset > Erase All Content and Settings). However, there's still a fast way to erase photos efficiently instead of removing them one at a time.
Open the Photos app on your iOS device, the make sure Photos is selected at the bottom of the screen. Click on a year, then a collection; you'll see your photos grouped into Moments, which clusters them by date. If you hit the Select button in the upper-right corner, you can then select each grouping by tapping the associated Select indicators that appear. You'll still have to select each Moments group manually, but it's a much faster option than selecting each photo individually. Once you've selected all your Moments, hit the trash icon, then confirm that you want to delete the photos, and they'll be whisked away to your Recently Deleted folder. To remove them for good, go to Albums > Recently Deleted, then press Select in the upper-right corner, followed by Delete All in the lower-left corner. Now your photos should be permanently removed from your device.