I'm on ubuntu 14.04 using the default Nautilus file manager and the problem I'm having is that some of my jpeg images give a tiny thumbnail but others don't. It almost seem random. In addition is there a setting as to where I can click on the thumbnail and see a larger preview?
I do not know the exact answer to the first question. You can check Nautilus settings - the last tab which is about previews have some setting about size limit for thumbnail. You can try changing it.
In case of previews I've never seen any option which would let You see preview by clicking. However, You can use gnome-sushi or gloobus-preview with gloobus-sushi for previews after pressing space. It's pretty comfortable. Depending on Your system version You can try installing gnome-sushi just by typing following command in Terminal.
sudo apt-get install gnome-sushi
In case of gloobus-sushi, which is unavailable for Ubuntu 14.04, you can use commands below.
sudo add-apt-repository ppa:gloobus-dev/gloobus-preview sudo apt-get update sudo apt-get install gloobus-preview gloobus-sushi