How to Make a Picture rotate Continuously?

You could use RaphaelJS, since it's cross-browser compatible. For example, this code should work:

<div id="paper" style="width:300px; height:200px"></div>

<script type="text/javascript" src="(link to downloaded copy of raphaeljs)"></script>

<script type="text/javascript">
var paper = Raphael("paper");
var good_cat = paper.image( "http://pp19dd.com/_old/lily.png",40,20,96, 145);
var evil_cat = paper.image( "http://pp19dd.com/_old/cookie.png",160,20,96, 145);
var angle = 0;

setInterval( function() {
    angle += 45;       
    good_cat.stop().animate( { "transform": "R" + angle }, 300, "<>" );
}, 500 );


setInterval( function() {
    r = parseInt(Math.random() * 359);
    evil_cat.stop().animate( { "transform": "R" + r }, 300, "<>" );
}, 1000 );
</script>

See http://jsfiddle.net/AJgrU/ for an example.

a jsFiddle working example with CSS3 animations and transforms: http://jsfiddle.net/GcjKZ/1/ http://jsfiddle.net/GcjKZ/3/

img { position: absolute; width: 25px; height: 25px; 
  -moz-animation: 3s rotate infinite linear  ;
  -moz-transform-origin: 50% 50%; 
  -webkit-animation: 3s rotate infinite linear  ;
  -webkit-transform-origin: 50% 50%;  
}

@-moz-keyframes rotate {
    0    { -moz-transform: rotate(0); } 
    100% { -moz-transform: rotate(360deg); }  
}

@-webkit-keyframes rotate {
    0%    { -webkit-transform: rotate(0); }
    100% { -webkit-transform: rotate(360deg); } 
}

(sorry, my star is not really well centered)

@-webkit-keyframes rotate {
  from {
    -webkit-transform: rotate(0deg);
  }
  to { 
    -webkit-transform: rotate(360deg);
  }
}

img.star {
    -webkit-animation-name:            rotate; 
    -webkit-animation-duration:        0.5s; 
    -webkit-animation-iteration-count: infinite;
    -webkit-animation-timing-function: linear;
}

Add -moz-transform/animation, -ms-transform/animation, etc rules to support other browsers.

You could also make an animated GIF :).

Update

Animation support is available is most current browsers which means that the prefixes can be removed:

(For reverse rotation flip the 'from' and 'to')

@keyframes rotate {
  from {
    transform: rotate(0deg);
  }
  to { 
    transform: rotate(360deg);
  }
}

img.star {
    animation-name:            rotate; 
    animation-duration:        0.5s; 
    animation-iteration-count: infinite;
    animation-timing-function: linear;
}

Shorthand:

img.star {
    animation: rotate 0.5s infinite linear;
}

You can do that with CSS: (utilizing your GPU core):

img{ 
    animation:2s rotate infinite linear; 
}

@keyframes rotate{
    100%{ transform:rotate(1turn) } // or 360deg
}

<img src="https://ih0.redbubble.net/image.364229144.1663/flat,200x200,075,t.jpg">