How to delete photos from an iPhone or iPad running iOS 8
To delete a photo, find it in your Photos app and then tap 'Select' in the top right corner. Now, tap the bin icon in the bottom right corner and tap 'Delete photo.'
You can select multiple photos at once and then tap the bin icon to delete all of the photos selected.
If you've created your own folder of photographs and want to delete the whole thing, go to the Albums view and then tap 'Edit' in the top right corner. You can now tap the red circle that appears on the left to delete the album. You won't be able to do this with the default folders created by Apple.
In iOS 8, however, when you delete photographs by following the instructions above, they don't actually delete from your iPhone or iPad immediately. Instead, it moves the photos to the 'Recently Deleted' folder where it will remain for 30 days unless you go to the folder and delete it from there. This is handy when you accidentally delete a photograph, but if there's a photo you really want to remove from your iPhone or iPad you'll need to remember to delete it from here too.
All you need to do is tap 'Select' and then tap 'Delete All' or select individual photos.
It's important to remember that, if you don't use iCloud and you back up your iPhone or iPad via iTunes, your most recent backup might contain the photograph you've deleted. Back up again once you've deleted the photos you don't want to overwrite it.
How to delete photos from iCloud (Photo Stream and iCloud Photo Library)
If you use Photo Stream, which is an iCloud-based feature, you'll also want to go into your Photo Stream Album in the Photos app on your iPad or iPhone and then select the photographs you want to delete in the same way as described above. This will delete those photographs from all of the devices you use for Photo Stream.
iCloud Photo Library is still in beta so not all of its features are active yet, but you can turn it on if you're using an iPhone or iPad in order to store your photos and videos in the cloud, access them on all of your connected devices and save space on your devices.
If you use iCloud Photo Library, you can delete photographs by going to any of your connected devices and deleting it in the same way as described previously in this article. This will delete the photograph from your iCloud Photo Library so they will no longer be available on your iPhone, iPad, iPod touch or web browser through iCloud.com.