When taking screenshots you need to ensure a couple of things.
The image should always be captured at the size you want to use and using formats such as png or gif are preferred over jpgs as the jpg algorithm will interfere with the look and feel. Screenshot should never be scaled either bigger or smaller as this will affect what they look like. If you are using a 720x405 slide size then you should not capture an image bigger that this.
Storyline prefers 96dpi images and some capture tools capture at a lower setting, these will be changed to 96 dpi on publish.
Finally check the output settings as image quality can be downgraded on publish.