Do evaluates its arguments but does not return a value.
Although Print is a convenient hack to get a graphic (or something else) output from a Do loop, it is strongly discouraged. Print's main use is as a tool to employ in debugging. If you want to see the graphics that you generate from within a Do loop you should do something like assign each of them to a parameter or place them in a list that is assigned to a parameter and then execute that parameter to return the graphics in an Output cell. Or you could create a formatted expression with those graphics in, for example, a GraphicsGrid or a Grid or a Column.
Here is an example:
plots = {};
Do[AppendTo[plots, Plot[x^n, {x, -10, 10}]], {n, 1, 5}];
Column[plots]
