How do I expand body conten width using Customize? ……without using CSS.

Hello there,
I’ve looked around in this forum for the past hour and could not find my answer anywhere… (more like, I couldn’t understand anything that might have been the answer to my question)

My question is actually fairly simple – most of my photos in my blog are 1024px*768px and I want them to display that way, but because the theme is set to have a 700px width body content thus all my pictures have been shrunken to that width which is too small.

I have not purchased to edit CSS or installed any plug-ins and I don’t actually know too much about CSS… I have tried “Appearance > Customize” and tried changing the content width at the CSS tab, but nothing happened…

I really don’t want to venture too far into the field of CSS (or PHP or whatever they are…) I really just wanted to have a simple, pretty template to share my photos and travel journals without having to fiddle around too much with it. Please could you help?

Many thanks.