Like the previous poster said, Silent Image is good for creating illusory barriers to the line of sight. Doors or Walls that aren't there, etc.
Alternatively, though:
It can also be used to create illusory passages where there are none. Ever see watchmen run full speed into a wall they thought was an open gate? Priceless! (Works on PCs too!)
The guardsman you just mugged and dragged inside the room? Why, he's still standing guard, silent and stoic as usual...
The closet in which you're hiding from the husband of the woman you were with? Full of clothes.
The 100 carat emerald you stole is still sitting there on display, until you're clearly in no position to be a suspect..."Who me? I was across the room talking to the Chief Archivist of the Library!"
In other words, its used to make things that are there seem as if they weren't, and vice versa.
Um, in case you haven't guessed, I tend to multiclass my illusionists with rogues or team up with them...