For cropping and resizing I'd recommend XNView: http://www.xnview.com/en/xnview.html

Remember to crop away everything other than the raw image as the example in my first post:
vs

As some simple guidelines for resizing, very high quality pictures from HD sources can be as much as 960px wide or 498px tall. DVD quality should be 720px wide if in widescreen. In cases such as this with youtube videos I'd say they probably should be 640px or 600px, unless they are very good quality.

Make sure the ratio is right too. See if the cars appear too tall or too long (are the wheels round?). If they look fine, then upload them as they are. As general guidelines:

Widescreen: 640x360 or 720x405.
Older non-widescreen (like those pics of the Volga) 640x480 or 600x450.

However, if the pics look fine ratio wise but don't quite match these size, don't worry. So long as they look right.

Compare:
vs

To see what I mean by this. The second pic as at a wrong ratio.

Thak you very much, guys, I will try, but not today and not tomorrow, I don't have the courage to try again, yet.

Indeed, it provides the image in the original size. On DVDs the picture use non-square pixels so they are not stored in the same format as they are displayed, hence why they have to be resized. But since you always have to also remove black borders, both can be done at the same time in xnview.

The problem is that when I press PrtSC I obtain nothing (I tried on YouTube).

PrintScreen key is not the best way to make captures anyway
For Youtube, the best solution is to get the file using JDownloader or http://www.bestvideodownloader.com/ for example (I use their Firefox Add-On)
This allows to get the highest quality available.
Once you have the file you can make captures with the programs mentioned previously (MPC-HC or VLC for example).

Thanks!

YTD Downloader is also good for downloading from Youtube.

http://download.cnet.com/YTD-Video-Downloader/3000-2071_4-10647340.html

Thank you, Raul!

Is it possible to highlight a certain part of the picture to identify the car of interest? I have seen this several times but cannot seem to find the option to do this. Thanks

There is. Once you've added the picture you can return to the edit page and there will now be a Car Location box (second from the bottom). If you click on the Select on Picture link you just need to click on where the car is located on the picture and a box appears around the car.

Thank you Sandie