Could you show a screenshot showing the problem with "B_Bigforest" in the game? Also, if there are any flowers nearby, screenshot those as well. That's because "leafpalms.tga" has different versions in different subfolders of the "Textures\Locations" folder, and that's because "leafpalms.tga" also contains the textures for flowers. Some time ago I brightened up the flowers and made different versions with different colour flowers for each location, so you'll see red flowers normally, but blue flowers if the location is taking its texture from the "ENGLAND" subfolder, and other colours for France, Holland and Spain. There's also a "JUNGLE" subfolder; if you see pink flowers, the location is using that. So the flower colour will tell me which version of "leafpalms.tga" is being used. (Having said that, there shouldn't be any difference to trees because all I changed was the flower part. But they probably could all do with some work anyway because if I look at "B_Bigforest" in GMViewer and point it at the texture file from the original game, I can see brown coconuts which either don't show up at all, or show up green, when I point it at the current texture.)
In any case, changing a file to use "leaf_icon" instead of "leafpalms" probably won't work because the textures are laid out differently.