Here is what works for me on all tested browsers and mobile devices (Chrome, IE, Firefox, Safari, iPad, iphone 5 and 6, Android).
The key for safari (including ios devices) is to add the other transform css rules and not just:
transform: translateY(-50%);
You need to add to it this group of rules:
-ms-transform: translateY(-50%);
-webkit-transform: translateY(-50%);
-moz-transform: translateY(-50%);
-o-transform: translateY(-50%);
Here is some working code of mine:
img.ui-li-thumb {
position: absolute;
left: 1px;
top: 50%;
-ms-transform: translateY(-50%);
-webkit-transform: translateY(-50%);
-moz-transform: translateY(-50%);
-o-transform: translateY(-50%);
transform: translateY(-50%);
}