Here is the official ZIM icon and logo for ZIMjs. Note: when in ZIM you can use frame.makeIcon() to make the icon. There are parameters for colors too. This is preferable as the icon is vector. But for HTML pages you can use these icons and logos.
ZIM Icon for Light Backgrounds:
ZIM Logo For Light Backgrounds:
ZIM Background Transparent Icon For Dark Backgrounds:
ZIM Background Transparent Logo for Dark Backgrounds