instant meditation - Imagining things and people in their decayed/old form? When I see something or someone beautiful, would it be good to imagine that thing or that person in decayed/old form? This idea came to me when I was reading about "Dasa sanya" (i.e. dasasaññā or "ten perceptions").

Will it be bad practice to take "Vipassana meditation" in a mental image form, like focusing on the decayed/old form of the particular object or person, instead of focusing on what I see?
My general opinion is that this would be one way of reflecting on the drawbacks/dangers of the object--namely, the sense-object perceived as beautiful is subject to aging and decay. This would be one way of attending to the sense-object that is producing lust as 'unattractive' and as not being ultimately satisfying. That can't be wrong.

Also, I assume that you would do this in order to counteract lust; so, the question is: is it effective? If it actually reduces lust for that object then it is the right thing to do. 

It may be that it is not in line with some specific practices that are called 'vipassana', but I don't see why this would matter.