What changed in Android 4.4 KitKat with SD cards
It's simple, really. Prior to Android 4.4 KitKat, applications — provided they had permission to access the SD card — could read and write to any area on removable storage, including the system folders like DCIM, Alarms, etc. That has all changed, and now third-party applications — as in ones you download from Google Play or elsewhere — can only write to files and folders that they have created or have taken ownership of.
Google made things neater and more secure. Some apps have worked with that, many others are just broken.
This keeps things "tidy." Apps aren't dumping files everywhere on the card — something we've all encountered — and instead have one central location to put all their files. There also are some serious security concerns that were addressed by not letting an app write files just anywhere.
This means that Jerry's Awesome Photo Viewer app can still scan your entire system for images, build a thumbnail database of them all and save it to a folder on the SD card. But it can't move or save the pictures themselves to folders — including the Pictures folder — on the SD card because it does not "own" those folders. If programmed right, it could save copies of the pictures to Jerry's Awesome Photo Viewer's own folders on the SD card. The folder is part of the app, and if you uninstall it, the folder goes, too. The old method of putting anything anywhere you want is gone, forever.
The other side of the coin is that Jerry's Not Awesome App that steals your data no longer has write access to every file and folder on the SD card. Yes, previously any application that declared permission to read and write to the SD card was allowed to write files to any folder — including your system folders, and any folder something like a banking app might have made. Any type of file, too. That includes files that could be read when another app starts up and affect the settings or way that app works. It was a complete and total security nightmare, and why a lot of people — including yours truly — did not want phones with SD cards.