I think it would be easier to do the following:
Go to camera roll, click on select on the top right, individually select any photos on the bottom row (unless that row is complete, in which case skip this instruction). Then press the bottom right photo and drag finger across to the bottom left, then without lifting finger drag to top left corner and wait until they're all selected. It's important to start at the bottom as there's no function to scroll down while selecting, only scrolling up.