XF 1.5 Category icons

[Sunka]

I have PNG icons for categories (40 x 40px), and everything is ok with them, but when forum is resized to mobile size screen, icons seems to cut off in height. Any solution to have that icons resized in connection with screen size?
Maybe some another image extension?

.node.category.level_1.node_984 .categoryText {
    background: url('path/to/image/gastro.png') no-repeat scroll left center transparent;
        background-color: transparent;
        background-image: url('path/to/image/gastro.png');
        background-repeat: no-repeat;
        background-attachment: scroll;
        background-position: left center;
        background-clip: border-box;
        background-origin: padding-box;
        background-size: auto auto;
}

First image is normal icon, and second image is when screen is resized to mobile size.