First off it's probably a good idea to load the images ahead of time instead of reload every time you rotate the planet.
But in response to the question, Allegro has section for timers. The wiki has a little tutorial on how to make the main thread pause for a while, but you might find it a bit advanced. http://wiki.allegro.cc/index.php?title=Timers
Alternatively, you can use system-specific sleep functions to similar effect. I wouldn't necessarily recommend this, but it'll be easier to grasp.