No it is not. Some things just fall.
Not all naturally occurring phenomenon can or should be interpreted as a sign or omen. Unless the picture was of someone specific and of significance to you personally I would not take it too seriously. That said there is a superstition that says a picture falling from the wall, for no apparent reason is the omen of an imminent death, however, the glass in the picture must shatter for the omen to be believed as true.
The word omen is not a bad word
The nail was bad.
This can be a good or bad omen. If you don't like bees it is a bad omen however if you appreciate their role in nature it can be a good omen.
It's not an omen it is carelessness or bad luck.
In different societies and cultures, bats have been regarded by some as a "good omen" and others as a "bad omen".
It's a death omen
For their dark color and tendency to feast on carrion (especially after battles) crows are generally considered a bad omen
Yes an Omen can come in many forms.
It is not any kind of omen.
There is currently no evidence to support that a dead fox was either a bad or good omen.
It's a bad omen if you keep looking at it instead of the road.