Sometimes a 2D image of cloud just isn't going to cut it.
Maybe your game needs some fluffy, white, 3D clouds to fly through or there is some dramatic and dynamic change of weather.
In either case, knowing how to generate a 3D cloud programmatically is a good place to start.
A cloud particle texture is required for this recipe.
Since it will be replicated and rotated repeatedly to make up a cloud, any random collection of white or grey dots on a transparent background should do the job nicely, as seen here:
To start rendering your own in-game clouds: