

I don’t think you really answered (or even whether you were trying to).
I don’t think randomly going off-pitch is an aesthetic goal; I think if you wanted to do that you could easily, in an electric instrument, introduce a random pitch bend. No-one ever does… What people do do is introduce intentional pitch bends - vibrato is an obvious example, but also pushing the tuning of certain intervals outside what the typical equal-tempered distances would be.
The reason I asked the question above the way I did is because it seems universally acknowledged that intonation on the theremin synthesiser is significantly harder than on a fretless string-instrument, which affords the same expressiveness in pitch. Unless there is genuinely an advantage to its setup (and, again, randomly bad pitch is not one IMO) should we not want to make it easier?
The image editing part is already solved. The problem is the massive library of templates, which are copyright some unknown person, hence a legal liability. If you need to upload your own template you may as well use GIMP - it’s not like it’s hard to create a few text layers after adding the template. Prepositioning them and scaling them isn’t that big a deal.