Don't like it when chat is flooded with ponies? Too lazy to burn them all? You need PONIES: Protect Oneboxed Nuisance Images Everywhere Safely.
All oneboxed images are displayed with 0.1 opacity until you mouse over them.
Get the Chrome Extension
Simply download it and drag it into your Extensions page to install (you can drag directly from the download bar).
If you wish to use the styles raw with Stylish, here they are:
img.user-image{
opacity: 0.1;
transition: opacity 0.3s;
-webkit-transition: opacity 0.3s;
}
img.user-image:hover{
opacity: 1.0;
transition: opacity 0.3s;
-webkit-transition: opacity 0.3s;
}