Plug an Image Map shader with your image loaded into it into the Density Shader input of your cloud layer. Set the projection type to PlanY in the Image Map shader and set the size high - something like 10,000 or more. This is measured in meters. Your image will be used to generate the shape of the clouds and will be affected by the Depth and other settings in the cloud layer node. You may need to zoom out and above your clouds to see how your image is working and adjust from there. You might also try "Through Camera" projection, using a 2nd camera positioned above the clouds to project and position your image.
I think you will generally find that just using an image map shader will give you very unrealistic results so it is probably best to blend that with the default Density Shader that is created with a cloud layer. Use the Blend Shader input for this purpose.
- Oshyan