Macbook photo storage for digital art, strategies?
Actually, current 15" model MBPs are configurable with up to a 2TB SSD, and the 13" models up to a 1TB SSD.

As for your current setup, if your wife is often on the go then any online solution (be it iCloud or something else) may not really cut it. So, if it was me, I would just use an external USB drive. 

This is portable enough to take on the road, and to keep connected when at home. Of course, you'd want to make sure that you configure your backup regime to include its data in your backups, but this is really the easiest way to increase capacity.

Another option is to rearrange your data generally so that you use your internal SSD mainly for your digital art, and use an external drive for any other large amounts of data (e.g. iTunes music library). Of course this is just a matter of prioritising what's more important to you.

Finally, you may want to check what's using up space on the SSD. You may find that something unexpected is taking up a lot of space (e.g. com.apple.bird, etc). While you can try to determine this manually, an easier way to check this is to use software such as DaisyDisk.

IMPORTANT: I assume since you have a NAS setup at home you've already got a backup regime in place. If not, organise one now!