There are a number of tweaks out on the net that could have been used to disable the taskbar thumbnail previews. You might see if you may be able to use the tutorial below to enable them again.
Taskbar Thumbnail Previews - Enable or Disable
It looks like some sort of time delay setting for how long the opened icons stay highlighted is set to long. Most likely another feature in the program that was used to disable the taskbar thumbnail previews.
Hope this helps.