I'm sure Tom will be in here soon. I'm only making this suggestion in case he misses this thread. If he sees it I'm sure he will have a solution for you as he may have just missed this. What I'm suggesting will at least stop the images from stopping in the middle of a icon.
Those icons are White and the color can't be changed. That is why with a light Taskbar a darker image is used in this location. Actually screen resolution and amount of icons can control this so the size of the image is very important. A different size image can correct this or a transparent image. Tom may need to change the image size so it stretches all the way across this area or image. A image has to be in here so you can't just delete it using skinstudio.
What I would do at this point is change the Painting settings so the image isn't going across icons. Has to be done in skinstudio. If you open it in skinstudio go to Edit Horizontal Taskbars/Tray/Tray section. Click on Painting at the bottom. Change the Left setting to 100 and the Right to 0 and click Apply. This will at least place it over the Expander and make it look like a button there.
