General synonyms:
* Outlined
* Shadowed
* Profiled
* Contoured
* Framed
* Darkened
* Blacked out
* Against a bright background
More specific synonyms:
* Etched (against the sky) - if the silhouette is sharp and clear
* Fuzzy (against the light) - if the silhouette is indistinct
* Dim (against the sunset) - if the silhouette is faint
* Cut out (from the light) - if the silhouette is sharp and defined
* Projected (against the wall) - if the silhouette is created by light
The best synonym will depend on the specific situation and the effect you are trying to convey.