This will depend on the theme you're using. I checked http://andrewmilnephotography.wordpress.com/ and I see you're using the one column layout of the Twenty Eleven theme which is an option on the Appearance → Theme Options page.
To make the one-column layout wider in the Twenty Eleven theme, add this to your Appearance → Custom Design → CSS:
.one-column #page {
max-width: 960px;
}
Note that some images on single posts will probably stay at the size you originally inserted them. To fix that after making a structural change to your theme, you should adjust the size settings in the limit width option on the Appearance → Custom Design → CSS page as well as the image sizes on the Settings → Media page. Those settings will affect what size image will attempt to be used depending on what size you pick when the image is inserted. Once your settings are updated, then you can remove and re-insert images from your media library and they'll be placed with your new image settings.