Post pictures,larger than thumbnail

How do I increase the size of pictures that I include in a post.
Every time I add a photo picture they are much smaller than the original.
I want to be able to make the picture larger,rather than it always appearing as a thumbnail.
I click on ‘Browse’ high light picture in ‘My Pictures’then I click ‘upload’ then drag picture to the left upper corner of my post that I happen to be writing,then publish.
But the pic is always the same size,slightly bigger than a thumbnail.