If you are building a park, surely silver statues would be more fitting?
In any case, why limit yourself to a 'tree' design?
Create an entirely new metal 'tree'! Obsidian pronged horror, anyone?
Maybe it could even be a 'tree-house' and have upper stories with food stockpile, or tables/chairs etc.?
Possibilities are LITERALLY endless. That's DF for ya.
edit: I realise that this is not what you're asking.
The best way to do this is probably to use Adventurer mode with a flying (or climbing) protagonist to easily cross the map and look at all the different tree types. The best way to save data of lots of trees might be to use a recording either in game, or specific software.
Surely the best way to get an idea for what trees look like is to look at hundreds of trees in-game (forested biomes or heavily forested embark, for example).
Is it really necessary to capture, compile, collate, categorise, etc. potentially hundreds of images/files, of just trees? I imagine most trees are at least 5 z-levels, then editing, renaming, ordering etc. Even then, your results might not be representational. Of course, some people enjoy that kind of activity, and if you play DF you're naturally likely to view hundreds, if not thousands of trees over time anyway.
Not that trees aren't fascinating, they're just not the most fascinating thing in DF, by far. Is it really necessary, to post something that people have seen millions of times already? What would be new, or useful, for instance, about your efforts, that curious DF players don't already know?
Many players look in trees when there's something stuck up there, or to watch the blossoms fall. Trees on their own aren't very interesting.
And by far the best cross-section of trees in the game will not be a series of images, but looking in the actual game to see the trees in all their glory.
Further, your efforts might be outdated in a few versions if trees change.
Please, continue on your project for great justice and !fun! if you find joy in it, by all means.
It may be that you create/contribute something useful as the end result of all your efforts - I would never dream of stopping you in your endeavour to share content with others.
In another vein.... Why not fungiwood? Fungus trees are infinitely more fascinating in the DF mythos, imo.