ThingWorx's UI widgets are written using jQuery, so this is the approach I recommend for animating images. Section 9 of the Wiki contains information on creating widgets. Presumably, you could make some modifications to the existing Image widget to add animation.
To clarify, you can use the existing Image widget as the basis for an entirely new widget. You will want to copy the source files out $CATALINA_HOME/webapps/Thingworx/Common/thingworx/widgets, modify them and repackage them as an extension.