I was looking for a way to backup my photos online using Google Storage or Amazon S3. But I don't need (as a rule) to backup my .jpg files or the .psd files Photoshop makes. For disaster recovery eg the house burns down, the minimum is the .nef files.
One way I can do this is to create disk images containing only the file types I need
First I created a disk image of type 'sparse disk image'.
File > New > Blank Disk Image
give the image a name in the top of the box,
and change the settings at the bottom. Because this is a sparse image, it expands as you add more files and data to it, so even setting it to be DVD sized doesn't take up much space straight away. Think of it more as the maximum size you want it to get.
Then after clicking 'Create' you are asked for a password. Make it a good one! If for some reason you don't want the image to auto-mount on your own Mac, then you need to deselect 'remember password in my keychain'
then Disk Image goes and creates your image, and mounts it on the desktop for you
After that you can search for the files you want to back up, and simply drag them onto the mounted disk image
and here they are