Minor CSS changes to eliminate flickering of hover images

Hi Stella!

I have been trying to make the changes I mentioned in IRC, but had some trouble doing it with the default layout of the module. I did however make the change to the close button and to the navigation arrows of the alternate layout of the module. I did minor changes to the CSS and the image files to have the normal and hover states in one image, so that there is no flicker while the hover image loads. The problem I had with the main layout CSS is that the navigation links take up the whole image, so I couldn't hide the normal state when hovering the image. I did some experiments and came up with an alternative, I'll continue working on this and post any more possible changes to the CSS here.

Here is a list of the changes I made to the CSS files:

lightbox.css:
line: 188: changed background position from right to left.
line 199: changed background to background position.

lightbox_alt.css:
line 67: changed image name
line 73: changed image name and position
line 77: changed background to background position.
line 81: changed background to background position.
line 154: changed background position from right to left.
line 163: changed background to background position.

lightbox-rtl.css:
line: 188: changed background position from right to left.
line 199: changed background to background position.