In order to create a seamless pattern in Photoshop open the desired image and choose Filter > Other > Offset. Original image. Increase the values so that you will be able to easily see the offset and for Undefined area, choose Wrap Around.
Where is pattern in Photoshop?
Choose Edit > Define Pattern. Enter a name for the pattern in the Pattern Name dialog box. Note: If you are using a pattern from one image and applying it to another, Photoshop converts the color mode.