A:do you like a picture of me?
B:no, i like the one i have got.
A:Of WHO?

i want to know whether to use WHOM or WHO.

as the rule says WHOM should be used after e preposition it would be wrong to use WHO in that place.
please help me

You're right. It should be "Of whom?"

Quick tip:

It is no coincidence that 'whom' ends in the letter 'm' like 'him' and 'them'. You see, in terms of grammar, 'who' is like 'he' and 'they', and 'whom' is like 'him' and 'them'.

You wouldn't say, "Of he?", but you you would say, "Of him?". Therefore, 'whom' is correct.

Get it?

of whom....
=who is this?
-this is the teacher of CHRIS
=of whom?
-of CHRIS

so who goes to the teacher
(the teacher of Chris)
whom goes to Chris
(Chris' teacher)

Yeah, that sorted it.