Anna, if you want the folder to show a preview, double-click on the preview jpg and it'll probably open in preview, then select all (Command-A) and copy (Command-C). Close that file and go back to the finder. Highlight the folder and get info (Command-i) then click on the little folder at the top left of the info box and paste (Command-V).
One of the very best things about Macs is that virtually all of the most-used shortcuts are universal through all the major applications, such as the ones I used above, and the quit (Command-Q) command, close window (Command-W), save (Command-S). If only Photoshop and Illustrator used some of the same shortcuts for their commands, life would be close to perfect!!