Have you checked out Big Huge Labs? It's a free site that can do fun things with all of your photos, and you can pull your photos directly from your facebook albums or Flickr, or just your computer. The Mosaic Maker can create collages. I created this simple collage this morning of the pieces I made yesterday so you could get the idea. You can pick however many cells you want in your collage. No rounded corners, I'm afraid but still a good option in a pinch.
My collage isn't that big, but mostly because I used the html code directly for Flickr. Which means, if you click this image it will take you to the Flickr location. I think if you save the image to your computer, you can alter the size easily in your blog posts.