Loading image while fetching data

Loading image while fetching data

Apologies if this is a fairly simple question! I'm fetching data (from a MySQL database), and would like to show an animated loading image while the data is being downloaded, and obviously then hide it when the data is fully downloaded.  I've found plenty of tutorials describing how to achieve this is the other direction (i.e. when submitting a form) but I'm not sure how to adapt these to what I want. 

If anyone's got any suggestions / links to tutorials, I'd be most grateful.