It seems the chosen image is hardcoded as the first picture of the result set from picasa and cannot be changed via an option.
You will need to dive into JS programming to change the library according to your needs. Adding an option to control which of the items of media$content
instead of 0
is chosen might be a first start.
Maybe something like
var img_src = json.feed.entry[i].media$group.media$content[data.albumcover_number].url.split('/');
instead of the code line I linked above, and adding the default option here
'albumcover_number': 0
does the trick.