Why is WP resizing my blog post images?

I use WP 3.5.1 with the WPFolio theme. As an artist and photographer, I am setting up my WP to have both relatively static content, and a blog that is regularly updated. Navigation between content is handled through a menu bar at the top of the page. I have set up the top menu bar so that when you click on “BLOG” it links to all posts categorized as “blog”. The idea in theory is that every new post will automatically appear at the top, but I have yet to put it into practice as I am currently trying to figure out this issue with images being resized. I am not sure if this is the best way to create a blog through WP but it is the way I have figured out.

Anyway, my issue with images being resized…
I would generally like my photos to appear at a higher resolution (preferably 900px across along the horizontal sides). When I am writing the post and previewing it without any category designation, the post appears as I’d like it with the large images stacked on top of one another for viewers to scroll through. However, when I then designate the category as “blog” and publish the post, all of the images are severed on the right side by about 1/3.

You can see what is happening to the images here:
http://nathanielfink.com/category/blog/

and here you can see how I would like images to appear:
http://nathanielfink.com/2013/02/11/543/

Does anybody know how to stop this from happening? Of course I would like the full images to appear. I tried a workaround which was to go into the “Menu” page under “Appearances” and add another menu item that would link to the “Uncategorized” posts category, then redesignated the post in question as “uncategorized”. Rather than appear as a blog format, it showed the post as a thumbnail to be clicked on. This was not the desired effect.

I appreciate any help with this issue. Please keep in mind my expertise does not extend to coding. Thank you in advance.