Despite the ambiguity of the question, I think the answer is no.
Also the subject does not summarize the question as it was not
mentioned in the question which also leaves one to wonder if
you mean an image or a character.
You cannot place an image onto a sheet with conditional formatting.
You cannot place a character into a cell with conditional formatting.
You cannot test for the presence of an image over a cell with C.F.
(you would have to use code to determine corners of an image)
I guess J.E. came up with another possibility of the question,
diagonal borders, and a solution if that is what is meant.