How to override max-width for specific div?

Are you just looking to unset the max-width property? If so, try:

#pixel-perfect img {
    max-width: none;
}

Ensure that your #pixel-perfect img CSS declaration is BELOW the img, a img declaration, otherwise it will get overwritten.

#pixel-perfect img {
    max-width: none;
    width: auto;
    position: absolute;
    margin: -200px 0 0 -140px;
    z-index: -9999;
}

I think it's because a img is more specific than #pixel-perfect img. Element selector is more specific than id selector (So in this example you have 2 elements vs 1element and 1 id).

In order to fix it, you'd have to add an element to your declaration, for example:

a#pixel-perfect img {
    max-width: auto !important;
}