As far as I'm aware you can re- order images in the native photos app, which will then show in that order in a slideshow. Obviously you need to put all your images in an album then follow the instructions below provided by Diane B. I use photo apps a lot but only found out about this recently! 'I just tried it in one of my PHoto albums and it works fine. Open your album, click on the curved arrow on top right, then hold down the photo to be moved and don't take your finger off it but slide it to its new place. When you are finished rearranging, then click done. You will see the photo you are holding down with finger 'enlarge' a bit and be 'loose' and then you can just slide it into place. I just rearranged a whole album that way.'