That way you won't even have to pause the DVD. Pausing CAN cause a slight degradation to the on-screen image.
Run IrfanView first, setup and activate the screen capture routine to suit you, then run your DVD player.
At the required point, invoke the Screen Capture either once or many times. You can choose jpeg or any other file format you want.
When you close / minimize your DVD screen IrfanView will be 'holding' your captured files for you, or you can choose to save directly to a HDD directory.
The reason you have been unable to see the capture is because you have been capturing the background of the DVD image. The DVD image is not inherently part of the screen display - it 'floats' on the top of it, you have been capturing the shadow, so to speak.