We've got a semi-responsive site here. You can now have some pictures fill the entire screen width regardless of the device/screen size.
The pictures I'll use to do that will be 1920 pixels wide. The site content; text etc fits in an area that's 980px wide.
The below picture is 980px wide x 735 tall, this is roughly a 4:3 ratio and is the max size we'll display pictures of this type, eg. stock shots.
The yellow boxes illustrate the full screen width (1920px) and the middle box illustrates the 980px site area.
If this picture was taken with the a vehicle filling a 4:3 ratio, you can imagine that it wouldn't work for a full width image.
The picture I'm using as an example was taken considering that it was going to be used as a strip, so I left space top and bottom and focused on the VW thinking the left and right edges might disappear off screen.
Below I'm using the picture in a box that will increase in width to fill a larger screen, or be cropped on some smaller screens.
The effect that makes it look as if it is moving at a different speed to the foreground is called 'parallax'.
Set at actual size:
Set to zoom (most common setting, requires extra space at top and bottom):
In short. For site content I need high resolution pictures, nice and sharp, subject to fill the center third, with main focus in the center box. Content to left and right may/may not be visible.
For pictures of cars as stock you guys want to be working with something better than you have or check the resolution as the photo's are pixelated. Example of a high res. picture below (looks better dunnit?):