Why are the quality on my tumblr photos worse if I enlarge them in the html?

I have that 'Catching Elephant' on my tumblr as my theme, and I tried to make the photos a bit larger by editing the html, but then the quality of the pictures turned really bad. And they're not usually low quality photos, they're taken with a Nikon camera, and the pixels are usually around 3800x2800 (ca), which is why I don't understand why they look so bad now, just because I enlarged them a bit. Please help, it'd be very much appreciated!
Take a look at it if you'd like, it's http://accordingtoklara.tumblr.com/

Looks like Tumblr is re-sizing and compressing the pictures. This usually will cause distorting in the process. They do this to save space on their servers. Looking at the code it appears they are limiting the size to a 16:9 frame with around 760 pixels wide. You might want to re-size the picture to fit a little closer (not same size or smaller though). Keep the "frame" in a 16:9 orientation. If you don't keep the orientation Tumblr will probably re-size the pic and distort again.