If you mean the image that show on the workshop page: When you use the 'publish' option from in the arma 3d editor, there is a spot on the lower right of the UI window that shows up that lets you pick a jpg from your computer to use as the image.

Other than that, I don't do anything special when publishing. Just to add an image to a mission (to show in Arma when the mission is selected), you add those to the mission directory and either link to them in the description.ext file or add them through the 3d editor.

How do I publish a map with a thumbnail as u did ?