My opinion is that the Time Capsule should really be used as it was designed....for Time Machine backups....not as an "everyday" hard drive.
For one thing, access to files on the Time Capsule drive if you place movies, music, images, etc. on the drive is going to be very slow....and really slow if you plan to connect using wireless. Expect lag, stutters, signal drops, etc.
That brings up another question.....if you move your movies, music, images, etc over to the Time Capsule drive.....how will you back up this data? Surely, you plan to keep "original" versions on one drive and "copies" on another to be safe, correct?
My suggestion would be to add an external USB or FireWire drive and attach it directly to your Mac. Move your movies, music, etc. to that drive. Access will be very fast to your media this way. Highly recommended.
Then, Time Machine will backup both your Mac and the hard drive.....so you have backups of everything. Make sense?
Although I do not recommend that you move your files over to the Time Capsule drive based on the information that you have provided so far....If you still want to setup the disk image on the Time Capsule drive, follow these basic steps:
Open Macintosh HD > Applications > Utiltiies > Disk Utility
Click New Image at the top of the window
Make sure that the little arrow next to the Save As box is pointing up
Navigate through the Finder like interace to open up the Time Capsule drive
Name the disk image and select the size that you want
Click Create
Allow a very long time for the image to created.