You're better off moving pictures and videos to the SD card. The SD card is not a great solution for apps for many reasons. Another of our volunteers has a fantastic explanation that he copies and pastes to reply to these questions, he'll be along in a bit I'm sure. They're a good stop-gap until you can buy a device with more storage but it's not a long-term solution.
What you're probably experiencing is that when an app that's been moved to SD (not all of it is moved there, by the way) is updated it is reverted to internal storage. There's nothing you can really do about that as the SD card typically can't be set as the default install location on Android.