Flickr photo sizes

I’ve been using the Olsen theme and recently learned I can speed up my site by using my Flickr photos. I followed instructions to select my photo in Flickr, then click download, then click view all sizes, select the size I want, and copy and paste the image address to my wordpress post. It’s great because my images are a lot less blurry now.
However, when I choose the 1600×1067 size, it shows on my blog as the exact same as the 1024×683 size. I would like to display my photo in a larger size but they all look the same. How do I go about fixing this to display the larger size? My blog is kattertots.com if that helps.