I agree that this is a bit complicated, but I will try to guide you through :)
First select one of the question tabs from your timeline. Then switch to state mode where you will see 2 states - red and grey. Take the green hotspot aside for a moment and you will be able to click on the plus/minus image and the red tab. With the right click you can select 'Change Picture' and there you can switch to your new image. After that place the green hotspot back.
Unfortunately you can't replace all images at once, so you will have to go through all buttons to replace all images. Be aware that the buttons are duplicated (so you would have the nice transition when you open one tab) and you have to change the image also for the hidden buttons in the timeline.