Five seconds to show the image
This means that you have five seconds to show the image. The problem here is that the implication is that the user is going to be the one showing the image.
Five seconds are left to show the image
Same problem here. This means that the user has 5 seconds left to show the image.
Five seconds are left until showing the image
This doesn't have the same problem as before, but I still don't like this one. It sounds awkward. You could make it sound better by saying "until the showing of the image," but that is too verbose anyway.
five seconds are left until the image to be shown
This is missing a verb. It should be
Five seconds are left until the image is to be shown
However, this is too verbose in my opinion.
For something like counting down time until a puzzle ends, I find this to be common enough and not be too verbose:
Five seconds remaining until image is shown