I think you should add a class name for every option item, after your initial declarations, for example:
input[type=radio].button1:before {
background-image: url(img2.png);
}
input[type=radio].button1:checked:before {
background-image: url(img3.png);
}
[...]
And in your code you should use:
<input type="radio" name="radio1" class="radiostyle button1" value="1">
<input type="radio" name="radio1" class="radiostyle button2" value="2">
<input type="radio" name="radio1" class="radiostyle button3" value="3">