Hi there, Maybe I can help. I have the revolution church theme, that also runs featured content gallery, it took me hours to figure out, so here it is:
when you write a post, add the custom field "articleimg" then in the field, put the link to the image url (yoursite.com/image1.jpg) or wherever the image is located- and post the article.
then go back to 'posts' (where you see a list of all your posts) and hover your mouse over the post you just saved.- at the bottom of your browser (in firefox anyway) you'll see the post ID (a long link) ending with a number eg. post=431 remember the number (431 or whatever) and go to settings-> featured content gallery
in the settings, select "Select here to use individual post or page IDs"
now, all you have to do is enter the post number in the field below and hit save.
your image and post preview will now show in featured content gallery.
if you add extra posts, just separate them with commas (but no spaces)
and that's it!