To add full length side borders using images you simply need to nest one element inside the other and then apply background images to opposite sides. The outer div has the left border applied by repeating a background image down the left side of the element. The inner element has the right background image repeated down the right side of the inner element.
You can the just add some padding to keep the content clear of the borders. Here is a simple example:
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This is the content : This is the content: This is the content : This is the content: This is the content : This is the content: This is the content : This is the content: This is the content : This is the content: This is the content : This is the content: This is the content : This is the content: This is the content : This is the content: This is the content : This is the content: This is the content : This is the content: