Open a new image 200 X 200.  

Choose a gradient with contrasting colors or make a gradient of your choice.  For this tutorial I used this gradient:

Flood fill your image with the gradient.

Now open your Paintbrush Tool (dark color) and use these settings

 and paint a squiggley line across your image similar to this:

Go to Effects > Texture Effects > Tiles and use these settings (used the stained glass from the Presets dropdown and tile shape set at triangle):

Now go to Effects > Plugin Filters > Simple Filter > Pizza Slice Mirror and apply.

Now we are going to turn it into a seamless tile.  Go to Effects > Plugin Filters > 

Simple Filter > Quick Tile and apply.

You are all finished!

You can play around with the settings and different gradients and get different results every time.  No two ever turn out the same.   Have fun!

Hope you had fun creating your Stained Glass Tile backgrouond.   Please feel free to email  me  if you encounter any problems.