additionally, there should be special tiles with both trees available in some tilesheets - I know those exist for VXA, and that is what you state to use on your profile even if you posted in VX (which is a different engine than VXA)
You should be able to fix this by editing your tileset in GIMP/SAI/PS or any other image editor that lets you save an image with a transparent background. What you want to do is called "clumping", it's where you move your assets around and overlap them to make them look more natural.
After the initial success with one event, I set up four different events on the same map to detect who is in which slot in my current party, and fortunately it managed to work as intended, with the events changing image to match the party members even as I swapped their positions around in formation. This will be a great tool for immersive scene design.