Lost iPhone - Can someone import its pictures?

Long story short, I have been using my girlfriends old iPhone that she no longer uses, just with wi-fi since I don't have a smart phone.

Because it has no cell reception, I didn't activate "Find my iPhone" because it would be virtually useless without cell reception.

I believe someone stole it, but what I was wondering is if they could access my photos and upload them to their computer? I know when I plug my iPhone into my computer it automatically brings an option up to import all photos. Will this happen if they plug the iPhone into their computer? Will they be able to import all of my stuff?

It had a passcode if that is of any help. An iPhone 4s iOS 7