Hi Nick,
There does seem to be a problem as you have stated, which also occurs when published. (the issue is the thumb, as the image will still scroll completely if you use the scrollbar arrow buttons)
However there is an easy solution,
and that is to place the image in a scrollable Frame. Doing this has other benefits also, as you can place other objects within that scrollable Frame. So personally I would always scrollable frames anyway.
In regards to your "Touch Screen Presentation" -- you may like the ability of the user to be able to drag the image about within the scrollable frame. See the attached IMP for a mock up sample. (you'll need to use your own resource images and resize accordingly)
One thing which would be good, is if the image would not drag beyond its boundaries. Perhaps some clever OpusScripter can come up with something here.