In this tutorial I'll explain how to get a screen cap from a DVD using Cyberlink PowerDVD for the capping and IrfanView to convert the files.
IrfanView is a free product, but PowerDVD is not. Many computers come with a free copy of this software, but as far as I know those will not make screen caps. If you're not sure if your copy will cap, then its best to download a trial version of PowerDVD at the link above.

Okay, so let's get started.

First, open up PowerDVD and right-click on the program. Select 'Configuration'.

When you've got that, click on the Player Setting tab, and select 'Advanced'.

Amend the settings to fit what you see below. Make sure the 'Capture to File' button is selected and that you pick the folder you'd like the caps to go to, and that the 'Keep Aspect Ratio' box is checked.

Once these settings are amended, it's time to start capping! :)

To make a screen capture, all you have to do is hit the 'C' button on your keyboard. Usually you can get a clearer screen cap if you pause the DVD (just hit the spacebar) before you capture the frame. To go through a scene frame by frame, you can hit the 'T' button. That will make you go forward a frame; I'm not sure if there is a button that will send you backwards a frame.

Once you're done capping, you'll have a folder full of .PNG screen captures. This is okay in and of itself, but Adobe Photoshop does not like PNG's for some reason, plus the file size is huge. So you're going to want some way to convert all these to JPG's quickly and efficiently.

Open up IrfanView and go to File > Batch Conversion/Rename.

Go to the folder where your caps are stored, and select them all and click the 'Add' button on the left. Then make sure that the output directory folder is the one you want, and amend the settings to match the above image.

Click on the 'Set Rename Options' button below the JPG drop-down list, set the quality bar to 100 and make sure the bottom four boxes are checked.

Now, go back to the main window and click on the 'Set Advanced Options' button. Here is where you can specify the way your files will be converted...the most important thing to do is to set the resolution to 72. I like to check the 'Delete original files after conversion' box, but that's just a personal opinion. You can also use this window to crop the caps you have (if you're really picky about getting those annoying widescreen black bars off) if you want to.

Once you've amended all the settings, simply click the 'Start' button on the left and wait for the files to convert. A dialogue box will pop up and give you a log of all the tasks performed. If things start running slowly (the program converts fairly fast) then you might want to move some of the caps already converted out of the destination folder. I've heard that once the cap total reaches over 1000 per-folder, things start to lag.

Well, that was the whole tutorial. I hope that was coherent enough. :p

Note about aspect ratio: You may have noticed with other screen captures that the image is somewhat squished from side to side. This is probably because the aspect ratio is off.
There is a lot of complicated math behind it (and I don't fully understand it myself so I'm afraid I can't help you there!) but basically it's a set height and width, and while you can increase or decrease either, if you don't do them together then your image will seem skewed.
Widescreen screen captures are roughly 5 to 2 which means if my image is 500 pixels wide, it needs to be about 200 pixels high to keep the ratio correct.
What does that mean? lol...I really don't know. But just a tip...if your image seems squished...your ratio is off. Try increasing the width or the height ALONE of the image, depending on what part of it is messed up.

Enjoy!