One question I get asked a lot is how I get the pictures so large on my blog. Let me show you.
I really like large pictures on blogs, and unfortunately Blogger doesn't really give you many options for sizing pictures. Here's what a photo uploaded into Blogger's "Upload Image" tool looks like:

A little too small for my taste.
So the first thing I do is go to flickr and upload my photos to flickr. (flickr is a free photo sharing site, and a super cool resource.)
Once the photo has been uploaded to flickr, click on the photo you wish to put on your blog. Then click on the "All Sizes" button above the photo.

After clicking on the "All Sizes" button, flickr gives you several photo size options. I almost always choose the 'large' size. Then, after choosing your photo size, copy the photo's URL,

then go back into Blogger into the 'upload images' button, and paste the URL here:

And there you go!

I like that much better.
This might have been clear as mud, so if I didn't make any sense, let me know. And if you put up a big 'ole picture on your blog, show me!******************
(Also, I should mention that you'll likely need to tweak your blog's html code a bit to make your margins wider to acommodate the larger pictures. This is where my limited knowledge gets even more limited. I just kept changing the margins little by little until I got the size I wanted.
Here and
here are a couple of links that may help with that. Hope that helps some.)