I'm confused - are you looking for something to back up your photos, or are you looking for a solution that can be used to transfer them from a PC to a Mac somewhere down the road?
If you're just looking for a backup solution, then anything would work (even an SD card, they are available in sizes up to 128GB. A 32GB card -- or an equivalent thumb drive as others have suggested -- would work perfectly).
But if you also want something that you can use to transfer these photos to a new Mac, you'll need to find something with dual-platform capability (so that the PC can write the files and the Mac will be able to read them later).
There aren't many of those. In particular I personally have not found any thumb drives that will transfer files between Macs and PCs (could be some out there, maybe someone else has seen one???). But there are a few external drives that will: Apple's Time Capsule is one, SeaGate Go-Flex is another. Both drives can be fully accessed from either a Mac or a PC, and files saved from a PC can be opened on a Mac, and vice versa.
Many cloud services also let you do the same thing: upload files from a PC, download them to a Mac.