How to add a background to the topic??

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Hello Bolgers, |adding a background to the article| What I mean is, NOT making a general background to whole blogg. I found some bloggers adding different backgrounds to each topic. How to do this?? Ok ! after designing the background by photoshop, and after writing a topic (any article for example), THEN WHAT!! Now I want to add this background to the topic. Only this topic not all messages in the blog. I don't want to do anything with the template. Just the topic. I hope I cleared what I want to ask about ^_^