document.getElementById('myImage').style.transform = "skew(-15deg)"
The «skew» property of the «transform» CSS method takes degrees of skew as an input. Negative numbers will skew the image top right, bottom left and positive numbers..the other way around.
Note that in order for this to actually work, you will need to use vendor prefixes as the time of writing this post. For Safari (desktop and iOS) and Chrome (and Android) you would use «webkitTransform» for FireFox «mozTransform» and so on.