Sketchup isn't really designed for this kind of stuff, especially in 2D games if that's what you mean by isometric-taking screenshots just to save an image would be quite tedious. I would recommend using one the of the paint tools here:
http://www.java-gaming.org/topics/pixel-art/28530/msg/259740/view.html#msg259740There's a short and sweet tutorial on drawing isometric stuff here:
http://gas13.ru/v3/tutorials/isometric_pixelart_tutorial_mathematics_of_isometrics.php - it's for photoshop but I'm sure you could do it in MSPaint or whatever art editor you prefer.