I'm no expert on this but I got a few emails asking me how to make pictures bigger on your blog. I just recently learned how to do this (kudos to Karol Jeanne!) and will do my best to explain. If you upload your pictures to flickr and photobucket then it's even easier. You just choose large as your file size and import that photo code in the "Edit Html" section of the post. I don't use a 3rd party for importing my photos to blogger so this is what works the best for me. And don't let all the steps scare you. It really does just take seconds to make these changes.
1. Upload your picture just as you normally would do in blogger.
2. Then while in your post, click on "Edit Html".
3. Look for the width and height of that picture (circled in yellow). And delete that whole section of the code.
4. Now you can change the size of your picture. Try a few different sizes to see which one you like best for your blog. Bloggers biggest upload size is s400. You can try s576, s640 or s800 depending on the look and settings on your blog. (You may need to change your blog template settings to accommodate the larger photos. Do this by opening your blogger template, choose "edit html". Scroll down to where it says outer-wrapper and main-wrapper. Change your "outer wrapper" width to 1200px and your "main wrapper" width to 900px. I had to get rid of my cute background paper and use a layout that only has 2 columns like Minima white.)
It should look like this to begin with:
Delete the 400 and put in the new number. It will now look like this:
And you'll have big, beautiful pictures. Or if you're not a very good photographer, you'll just have big pictures. Which was the purpose of this tutorial. Good luck!
Example pictures at different image sizes:
The original size of a large image uploaded on blogger (s400)