we're using two images with a hoover effect, one smaller and one somewhat bigger. But when I hoover the image the larger one is really poor quality, why? http://adata.worldwebmastersforum.com/
You should never resize images using css. Instead, just crop the image to the size you need and use it that way. Images resized via css will never look good quality, and can be detrimental to your page load times.
It's because you're doing it wrong! You can't change a background image of an img, the original img will still be above the background. You need to set the img as a background image on the <a> tag. To be honest though, it isn't a very nice effect when things resize on hover, it makes all the surroundings move around and is generally quite annoying. Try using sprites - have a read of this tutorial - http://www.ehousestudio.com/blog/view/css_sprite_navigation_tutorial