To start with, I like to utilize the public/ folders in my packages for storing these values. That way, if you put images in any public/images/ folder (I put a public/ folder in every module), they will end up in your war in the images/ folder.
First make sure that the above is working correctly for you. You can also utilize an ImageBundle which will assure you the images are available through compile time checking. I find this the best solution.
With an ImageBundle your rule becomes:
@sprite
.rule1 {
}
To investigate this further use FireBug or Chrome dev tools to find what image resource your .rule1 is pointing at. That will probably reveal the problem.
Sincerely,