It has to do with the way the iPhone structures its internal memory for the camera roll. You know, the DCIM folder that you have with most modern devices that have a shooter attached to them. With most devices, like ones on Android, it's one folder for all the images, and all images within are sorted by the date they were taken on. Not so with the iPhone.
On an iPhone, you get a new folder created for every 100 photos you took. Note that we say 'took', not 'have', so even if you deleted older pictures, this logic will still apply mercilessly. The result? If you're a trigger-happy photography buff and have shot thousands of images on your iPhone since you last set it up, you can easily end up with tens of folders, even hundreds! Worst of all? This is the type of cryptic names they get:
Notice that these alphanumerical tags give no identifiable information as far as we can tell, so you can't apply some logic to distinguish between what's new and what's old. And, no, you can't sort the folders by date, as they all have identical 'last modified' dates. Even worse, there's no separate folder for screenshots, of which we take many, so you have to hunt them down manually every time you need to download images to your computer.
What a mess! And we're not alone in our displeasure with this 'arrangement'. Sure, you could use iTunes, but that's not an ideal solution – after all, iTunes is a relatively heavy piece of software (doubly so on older machines) that needs time to sync up with your phone, and it is possible that you're trying to upload images on a different machine that doesn't have iTunes on it installed.
Apple, this needs to go.