That is how iPad Stock Photo app is.. Of course I learnt it hard way but there is no way you can do sequencing or keep pictures in order. You would need to use a third party application for this. I use an app named Sortshots (
www.sortshots.com) which lets you sequence on name, size etc. as well as create a custom sequence with a simple drag and drop interface. There are several more features like EXIF/Metadata support, Tagging, Share and so on but the sequencing is the one which made me buy this thing.. Take a look in case that helps