What if your blog posts are on different topics? Then you can put a different picture as the background for each post.......
SOURCE THE PICTURE
First step is to search for an appropriate picture for your post. Use Google to search for copyright free images.
UPLOAD PICTURE TO FREE HOSTING
Next step is to upload the picture to a free web host and copy down the link. You can use Googlepages or Photobucket.com for this.
PUT CODE IN POST TEMPLATE
1. Login at Blogger.com.
2. Click on Settings link on the Dashboard.
3. Click on the Formatting subtab of the Settings tab.
4. Scroll down in the Formatting tab to the Post Template at the bottom.
5. Paste this code at the top of the box :
<div style="background:url(LINK_OF_PICTURE) no-repeat;">
and paste this code at the bottom of the box :
6. Save Settings.
7. Click on Posting tab at the top.
8. The Post Editor will open with the code already in it.
9. Replace 'LINK_OF_PICTURE' with actual link of the picture uploaded to your free host. If you do not want the picture to repeat delete the "no-repeat" in the code line.
Type in your post before the </div> tag at the bottom, give it a Title and Publish.
See the example in this post.