Google finds only thumbnails from my webpage?

Lille Ulven

Hi.

While I have some time ... well not really but ... I am trying to figure out how to make my website's photos better findable by google. So far so good.

Now I found that if for example I use the following search terms:
"whanganui river valley lille ulven" only one of the first 30 matches is actually a photo from my website larger than the 100x100px thumbnail. All the other larger photos are taken from external links like my blog, pinterest or similar, while almost all my website matches are thumbnail size...

Is there anyway to explain to google to not go for the thumbnails but the larger photos shown on my website as well? (Without changing from smug mug style to something different, as I really find smug mug style the most "informative" and best looking style available)

Thanks in advance

Lille Ulven