Catherine, the high resolution photos you upload are kept on a server in a location that's not available to the viewing public.
FAA generates a 900 pixel tall version for the image page and smaller versions for thumbnails, etc. Your work is safe. Upload the highest resolution images you can.
If you share your work on the internet, it will probably get re-used by someone at some point. But the only thing that will be available from here is a relatively small image, and since FAA prohibits right clicking on the site (to make it hard to "save image"), it takes a little effort for a person to take your preview images.
It's a tradeoff. You want to show your work online, you have to be ok with people sharing/using it. You want to sell, you have to show them large enough previews that they can see detail. True theft—images taken for the purpose of re-selling elsewhere—isn't all that common.