If you save images on the Device Memory, you need to really transfer those to the Media Card. Move device memory images by highlighting the thumbnail of the image in the Media icon, click Menu > Move, and navigate to your Media Card to drop.
And I don't use Media Manager, to me to limiting and just does do that good of a job anyway. Read this link for another method, using Mass Storage Mode on your Media Card and treating it simply as a removable folder on your BB. How do I add media files to my microSD card? - BlackBerryFAQ