Here's what I mean (images are at 50%).
Given this original image:
- Create a Lasso around the important part of the image. I used the Quick Selection tool and Mask Brush:
- Use Tool...Lasso...Convert Selection to Layer to place that part of the image on its own layer at the top of the stack.
- Activate the bottom (original image) layer, and apply the Effects...Special Tools...Generate Pattern.
- Add a Curves adjustment layer above that patterned layer and darken it. I used the Lab colour model to reduce the Lightness without affecting the colours:
- Use Edit...Merged Copy to copy the result to the Clipboard.
- Open the Pattern list and paste the Clipboard contents in as a new pattern.