Promote the background to raster(if you have not already done so).
Select the part of the image you want to keep using your lasso (selection tool). Go to selections/invert and press delete -- this will remove the old background. Go to selections/select none.
Add a layer and go to layers/arrange/send to bottom. Now choose the cloud image you want as background and put it in your pattern box of your background, then choose flood fill and left click in the empty layer you just added.
If you don't like it, click redo and play around with the settings in the pattern box until you do have something you like.
Merge layers/flatten and save as a jpg.
Margaret