What do I do to get to the next image to index ?
Does the indexing batch have more than one image in it? (Not all of them do.)
I've had no luck getting the Help Center to cough up anything about indexing using a tablet or mobile device. From what I can tell, you use the website, same as on a computer, so getting to the next image (if there is one) should be the same: you can use either the arrow on the right-hand edge of the screen, or the "Next Image" button at the bottom of the text entry area when you're on the last entry for the image.
If what you see/use on a tablet is completely different, then I apologize for the useless advice, but I don't have access to a tablet to try it on (because if I did, I would likely throw it across the room in frustration for reasons completely unrelated to FS).