Is there an instruction to direct the loading and ‘playout’ of a still image for a given duration?
No. There is no AMCP command that does this for you.
You need to handle this at the client side.
A common trick on the official client would be to group the image together with a playout command “Stop” which is delayed by the time you want the image to be shown. If you want a soft transition you could instead use a transparent image (f.ex. the solid color tool) and mix it in on the same layer.