I know this has been raised before, without real resolution.
I am wanting to create a trigger that fires when Image is created ( under a specific root node) ,
that image is set as a thumbnail of the direct parent asset.
Thumbnails are set via NOTICE links, so you can use the Create Link trigger action to fire when the image is created to create a Notice link to its parent using the dynamic parameter, like this:
You can use that as well, I guess you would then use Action on Other Assets and select the %asset_parent% as the asset to fire on and use the current asset as a dynamic parameter source for the Set Thumbnail action.