Show[Image[xx]] doesn't help either. Image[xx] works fine outside the For loop.
Thank you, Frank, that worked! It seems to me that Plot and Image should have an implicit Print.
Put it inside Print. For example
For[i = 0, i < 3, i++, Print[Plot[x^i, {x, 0, 1}]]]
However, For loops are slow in Mathematica. Better to use Table
