If you're using WinPE 5, you will need to find the Windows 8.1 driver for the model computer you're trying to capture an image from. Once you have this, go to Tools > PXE Configuration from the console. Then select the WinPE boot menu option and click Edit on the right. Click the Edit Boot Image button on the next screen. Click the "<< Edit" button. Click the Next button. Click the Have Disk button. This is where you point it at the driver you downloaded. After you've added it, verify it shows in the list. The click Next until it regenerates the images and then click Save once back at the PXE Configuration screen. Once this finishes, you should be able to boot and capture the image.