To use PhotoShop Brushes in PI you will need to download this ABR Viewer. This viewer will convert .abr files to .png

There is a great tutorial Here to learn how to do this.

Once you have done this the file/s will then be needed to be changed into .jpg files.

Open in PI . Ctrl+D to Duplicate the original. File/Save As. File Type jpg. Give it a name then save it to where you will be able to find it again :)

To then use your new brush in PI select your PaintBrush Tool.

SETTINGS:. Mode/ Texture/ Add Texture. Browse to where you saved the file and double click on it.

PANEL: Set the size of your Brush to the size of the image you saved. As you will be painting in Mode you will be able to resize or move this result later. Set The Colour.

Click once on your canvas where you want the brush to be used.

Clicking again will deepen the color but if you want to do this make sure not to move the brush!

A couple of Great sites I have found for Free PhotoShop Brushes:

http://www.blendfu.com/

http://psd.tutsplus.com/articles/web/50-insanely-weird-but-awesome-photoshop-brushes/