Hello,
While it's difficult to know what your program is doing without seeing a screenshot or the vi itself (hint, hint), the behavior you describe sounds like you are taking the graph's image without ensuring that the graph has actually updated with data first. To fix this, you can enforce dataflow dependency between the two operations (connecting error clusters is a very useful tool here) and introduce a small wait function before taking the image to allow the front panel to update. Highlight execution mode will result in different timing for your code, meaning that the front panel may now have enough time to update fully before the image is taken.
If you could post a screen shot of your block diagram and a little more detail about what you mean by being unable to get the image (do you get an error?), I'll take a look at it.
Cheers,
Matt Pollock
National Instruments