So if you upload 5 images, and they're named as such:
And you want the default picture to be the latter, you will have to rename it. To cut my babble short allow me to demonstrate
1. First click on the Simple Image button at the bottom of your article. This plugin doesn't come free, it will cost you about 15 Euros
3. Upload your pictures by dragging them into the drag box or manually upload them
4. Make sure you rename the first image to ensure that the picture you want is the first thumbnail image of the article
Of course, to make the articles appear in an uneven mosaic you need an altogether different module...