You may wish to read up on how libraries work, but the basic idea is that you can add/remove parent
locations to a library and all items in that location will be show in that library.
So, if you want your pictures to show up in the pictures library simply add the folders that contain those pictures. Be sure that there are no videos anywhere beneath the parent folders that you select to be included in the library. Do the same with videos.
In other words:
If you have an external hard drive that looks like this:
Add each location and only each location to the libraries. Do not store videos beneath the master pictures folder and vice versa.
How to Use Windows 7's New Library Feature > Understanding the Windows 7 Libraries
Windows 7 Libraries Explained, And Why You Want Them
Get to Know Windows 7 Libraries Inside and Out
Personally, I just place a link in the favorites list to my parent pictures and video folders. This allows me to access them like libraries as all of my data is already very well organized. In the simplest sense, libraries aid those who have unorganized files.