When adding on a new drive to an existing array that would be referred to as "Dynamic Disk Resizing". This is the main term to use when looking into that further.
First off you would need to convert the Dynamic volume to Basic before you can expand anything more then just the volume itself. If going to see this done in the Disk Management the items that are otherwise impossible to see done are expanding the boot volume, any mirror volume, increase a stripped volume, extending a Raid 5 volume, or extend anything converted from Basic to Dynamic.
With tons of photos filling up one drive and the need to add another on or go for a larger capacity you can actually see more done with fewer drives by selective measures. 2.5tb fits on a 3 or 4tb model drives still leaving some room for adding more on just one drive alone.
Compressed archives would take up even less drive space and can serve as a means of backup on a separate drive. When your first 3 or 4tb drive fills you then add the next on while the backup drive still has plenty of room left for archives for the second drive depending on the compression as well as backup method used.
To backup the first backup drive you then add in the 4th to back up the first as a separate logical drive. By having each drive separate from the others you can also separate groups of photos depending on what you are working on.
With an array on the other hand you have no backup plan inplace as well as everything on a single volume being extended across several drives. You might want to consider a home server type set up where you can continue to expand capacity without running into various limitations. Like MS pulling the Drive Extender function out of the 2008 R2 and Windows Small Business Server 2011 Essentials. Microsoft pulls Drive Extender functionality from Windows Home Server “Vail” | WinRumors
That was one tool available for extending file storage over multiple larger capacity drives being removed by MS there over security concerns mainly. As far as having one spanned volume or simply having several drives installed that's all still local internal access.
Main directories on each drive can easily hold countless sub folders to catagorise each set or sets of photos. But besides the risk factor with arrays to begin with not having a disaster recovery plan inplace especially since this is your livelihood should be unthinkable. What would you do if all 2.5tb of photos were suddenly gone?
This is why arrays always seem to best served on servers. A server setup for the ever increasing amount of storage needed could also have a backup plan at work. On a desktop being able to create and store images of storage drives also takes capacity needed for the destination drive where an image would be stored.
The concept for the home server would not only cover the growing storage need but also allow for file sharing if you have more then one local machine. Note the Window Home Server is a separate Windows from Windows 7. Windows Home Server gets Windows 7 support - Computerworld
The Raid controller used will also make a difference as far as allowing any additional drives to be added. Arrays are created across identical drives however as another item to mention here. You wouldn't be able to simply add a 4tb drive as one example onto an existing array using 2, 4, or more 1tb drives.