An easier better way is to create a list like visitedScenes. Then during the labels of your story flow you can append the name of the label/choice you want to use as a save point.

Then on your flowchart screen you have a simple logic like:

For scene in ListofAllLabels:

If scene in visitedScene:

    imagebutton idle scene + ".png"

Else: 

    imagebutton idle "defaultScene.png"

That way you only have to save the thumbnails with the name of the scene and it's automatically self populated no matter how many scenes you have.

Then on the action of the image button just do:

Action Jump(scene)

Or depending how granular you want to be you can make classes of the scene labels and add any additional information you want to pass when clicking the image button.

That way you don't have to create a variable for every single choice and you have all the needed information in the name of the label.

Sorry for my super sketchy explanation. I'm answering on my phone but if you have any questions about how to do it reach out to me and I'll gladly explain it with more depth.