I want to draw a graphic $20\times20$ grid, rotate it by $\theta$ around $(0,0)$, and trim off the part on the right of the y-axis, so I came up with
ImageTrim[Rotate[GraphicsGrid[Table["", {20}, {20}], Frame -> All], Dynamic[\[Theta]], {0, 0}], {-1000, -1000}, {0, 1000}];
But it gives no output, but an error message instead
ImageTrim::imginv: Expecting an image or graphics instead of [an oblique orange square wich usually indicates something's wrong]
What should I do?