For me, I think the 0.5 would work perfect
... but wouldn't it just be easier and make every one happy if the user was able to set the amount of time themselves?
When it comes to keeping it fullscreen or having it shrink, I guess that too would be nice for the user to choose. However, if the time can be about 0.5, I'm not sure if it really makes a difference if the image shrinks for that time or not.
I have also had another thought.
For images in fullscreen, I don't really think it's necessary to have the same LARGE toolbar as for when it playing music.
This way MC won't need a toolbar that disappears and re-appears.
I've made a mockup snapshot of what it could look like below.
The fullscreen display toolbar can also be customisable with shortcuts, similar to Music Centers Main window toolbar, where shortcuts like "next, previous, rotate, etc." , can be added like the user may want.
Fullscreen Display with Custom Toolbar and not so large