Show[Image[xx]] doesn't help either. Image[xx] works fine outside the For loop.
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
Thank you, Frank, that worked! It seems to me that Plot and Image should have an implicit Print.