Been playing with this and found a fun answer:
1) Export one of the tree item images to your desktop
2) Open in Gimp
3) Go to Image > Transform > rotate 90 degrees
4) Go to Image > Scale Image > make it any size you like ( click anchor symbol on right for complete control)
5) Save it
6) Open up VSB import your new image to replace the old one
7) Remove ContentMargins, SizingMargins and SizingType:Enum properties.
8) Add SizingType:Enum property with TRUESIZE value.
9) Test it to see what it looks like. Keep going back to Gimp to resize until it is what you want.
10) Import this image to replace all tree item images
11) Repeat parts 7 and 8 for all the images.
In this case the image is 150x12 pixels:
There's probably easier ways. But this does work.
