you can't PREload with varying paths.
As far as i know, this works like this:
preload loads before
_ready and just sets your texture, when i reaches the line.
You could use just
load, but this might give performance-issues, if you use this often.
you could preload all your textures in an array, and choose randomly one of them
OR (maybe the fastes)
you can create a global array (singelton) and choose it randomly from there. So it has to be (pre)loaded only 1 time.
OR (probably the best, thats what i would do)
export an array of textures:
export (Array, Texture) var textures
This has the huge advantage, that you can just drag&drop you textures into an array in the inspector.