You can do the same thing in 'Gimp', it's free and easy.
Here's a quick three sequence run down using Gimp, the more incremental images the smoother the animation, this is for anyone else you hasn't got animation capabilities in your rendering program or hasn't got Photoshop.
I'll use these three renders for example.
Open the first image and copy the other two images on separate layers, in the animation order.
Then save as (name).gif I used the name eye.gif for this example.
Once you hit the save button a option window opens, set it as shown for an animation.
When that's done the final option window will ask for a comment ( not really needed) the time between each frame in milliseconds, I chose 1000 millisec=1 sec and leave everything else as default unless you what to fiddle.
That's it! Enjoy.