I think you might be misunderstanding the Time Capsule a bit -- it's designed for use as a backup drive with Time Machine. You can certainly backup your Lightroom library to it for redundancy, but you can't edit photos off of it.
If you're running out of space and would like to add storage for photos, a 7200rpm FireWire or eSATA external drive is the way to go. You may even want to consider getting something like one of the WD MyBook drives with two-disks -- you can configure those as RAID 1, which gives you a real-time backup of all the data stored on the disk. Basically that means, if you buy a 2TB MyBook and configure it as RAID 1, you only have access to 1TB or storage space, because all data is mirrored to both physical drives in the enclosure -- if one of them crashes, you don't lose anything.
