This tutorial explains how to make patterns that will repeat seamlessly in Illustrator. It shows how to use single and multiple images.

Choose "Pattern" followed by "Make." Illustrator will enter pattern-making mode and add your pattern to the swatches palette. In the Pattern Options Palette, deselect "Move Tile With Art." ...

Once you've selected a pattern swatch in Illustrator, you can apply the pattern to other tools. Using the Rectangle Tool, you can quickly use a pattern to fill the entire page using the same pattern.