Enlarging images within blog post?

I’m loving Pinboard but I have a question to which I can’t find an answer…

On my blog entry (my first test), I would like the images to fill more of the white space (I removed links – I just want people to see the images in the post).

See: http://www.fiveinthehive.com/hello-world/

These are 300×199 but when I try to adjust/increase their sizes in the post itself, they just get blurry (these are high-res images – 2000 px). Where can I tell the page to size these differently?

Thanks for any help – I LOVE pinboard!

Nikki