You can just take a picture of some clouds you like (even better, string a few pictures together to make a panorama) and apply that to a sphere, putting your entire scene inside the sphere. then you just paint an incandescanse map with the sun as the most incandescent part, put a light by it(the sun) and parent the light to your sphere. that way you can even slowly rotate the sphere to imitate the movement of clouds. if you want to get really fancy, you can use a cloud video as the texture. if you want to make the sun more realistic, you can do a transp-mapped sphere for the clouds, a plane outside that for the sun, and another opaque sky blue sphere outside that for the BG and move them independantly of each other.