Image = ?

I dont get what the "Image" thing does in the trigger editor.
From the sound of it, it should be that you place an image on the terrain on desired spot, though that dont work for me.

Else what's the easiest way to attach a picture to the ground?

Use an image with a transparent border only (1 pixel at least) and set 'render always' to true. Use indicator image type since it suits perfectly most needs. More information in this tutorial :
Image tutorial