Indserting an Image

Hello. I am trying to insert a large image as a single post onto my homepage. If I use the ‘insert image’ function, the image does not appear on the page when I load the site up. Is this because my posts are limited to being a certain size?

Please visit http://www.manzatelacom to see what I mean. Where I have set a featured image of somebody ‘climbing’ a rock, this is where I want a large picture to be.

For the life of me I cannot figure out how to insert a large image into this space. Any help that anybody could give me would be great;y appreciated. Many thanks.