Upload the picture to a free web host and copy down the link. You can use Googlepages or Photobucket.com for this.
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(pic URL) 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 'pic URL' 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.