The sync method, by definition, means you replace all the pics, with all that you want. Mentally, you are using your computer as the master, and just copying down what you want. So if you really only want to just add specific pics, you cannot do that with the sync option. Although, once you get used to the routine, it is virtually the same. Assuming you want to add 1 picture, and assuming you have the photo section set up as you left it last time, you simply x the additional picture and push sync. It recopies everything that was there, plus the new picture.
But I can understand if you want to add and remove certain pics on the fly that you need a different solution. And Dropbox works great.
There are other options that might not fit into your free approach, but when you get a buck or two left over, take a look at photosync. I use that all the time to send picts to and from the pad, phone and desktop. Buy the app once and you can install it on any I devices. There is a desktop app as well, I think it is free, but not real sure, as it has been a year since I got it. And if you are using a brand x phone, there is a version for android. And they all talk to each other. Not free, but a worthwhile couple of bucks if you really want an easy way to flip pictures around. ( there are other similar apps, this just happens to be the one I am familiar with)