<a href="big.jpg" onclick="this.href='javascript:void(0);';">
<img src="small.jpg" border="0" onClick="new ImageExpander(this, 'big.jpg');"> 
Подпись, чтобы показать, что ничего не уезжает
</a>

function ImageExpander(oThumb, sImgSrc)
{
// store thumbnail image and overwrite its onclick handler.
this.oThumb = oThumb;
this.oThumb.expander = this;
this.oThumb.onclick = function() { this.expander.expand(); }

// record original size
this.smallWidth = oThumb.offsetWidth;
this.smallHeight = oThumb.offsetHeight; 

this.bExpand = true;
this.bTicks = false;

// self organized list
if ( !window.aImageExpanders )
{
window.aImageExpanders = new Array();
}
window.aImageExpanders.push(this);

// create the full sized image.
this.oImg = new Image();
this.oImg.expander = this;
this.oImg.onload = function(){this.expander.onload();}
this.oImg.src = sImgSrc;
}

ImageExpander.prototype.onload = function()
{
this.oDiv = document.createElement("div");
document.body.appendChild(this.oDiv);
this.oDiv.appendChild(this.oImg);
this.oDiv.style.position = "absolute";
this.oDiv.expander = this;
this.oDiv.onclick = function() {this.expander.toggle();};
this.oImg.title = "Click to reduce.";
this.bigWidth = this.oImg.width;
this.bigHeight = this.oImg.height;

if ( this.bExpand )
{
this.expand();
}
else
{
this.oDiv.style.visibility = "hidden";
this.oImg.style.visibility = "hidden";
}
}
ImageExpander.prototype.toggle = function()
{
this.bExpand = !this.bExpand;
if ( this.bExpand )
{
for ( var i in window.aImageExpanders )
if ( window.aImageExpanders[i] !== this )
window.aImageExpanders[i].reduce();
}
}
ImageExpander.prototype.expand = function()
{
// set direction of expansion.
this.bExpand = true;

// set all other images to reduce
for ( var i in window.aImageExpanders )
if ( window.aImageExpanders[i] !== this )
window.aImageExpanders[i].reduce();

// if not loaded, don't continue just yet
if ( !this.oDiv ) return;

// hide the thumbnail
this.oThumb.style.visibility = "hidden";

// calculate initial dimensions
this.x = this.oThumb.offsetLeft;
this.y = this.oThumb.offsetTop;
this.w = this.oThumb.clientWidth;
this.h = this.oThumb.clientHeight;

this.oDiv.style.left = this.x + "px";
this.oDiv.style.top = this.y + "px";
this.oImg.style.width = this.w + "px";
this.oImg.style.height = this.h + "px";
this.oDiv.style.visibility = "visible";
this.oImg.style.visibility = "visible";

// start the animation engine.
if ( !this.bTicks )
{
this.bTicks = true;
var pThis = this;
window.setTimeout(function(){pThis.tick();},25); 
}
}
ImageExpander.prototype.reduce = function()
{
// set direction of expansion.
this.bExpand = false;
}
ImageExpander.prototype.tick = function()
{
// calculate screen dimensions
var cw = document.body.clientWidth;
var ch = document.body.clientHeight;
var cx = document.body.scrollLeft + cw / 2;
var cy = document.body.scrollTop + ch / 2;

// calculate target
var tw,th,tx,ty;
if ( this.bExpand )
{
tw = this.bigWidth;
th = this.bigHeight;
if ( tw > cw )
{
th *= cw / tw;
tw = cw;
} 
if ( th > ch )
{
tw *= ch / th;
th = ch;
}
tx = cx - tw / 2;
ty = cy - th / 2; 
}
else
{
tw = this.smallWidth;
th = this.smallHeight;
tx = this.oThumb.offsetLeft;
ty = this.oThumb.offsetTop;
} 
// move 5% closer to target
var nHit = 0;
var fMove = function(n,tn) 
{
var dn = tn - n;
if ( Math.abs(dn) < 3 )
{
nHit++;
return tn;
}
else
{
return n + dn / 10;
}
}
this.x = fMove(this.x, tx);
this.y = fMove(this.y, ty);
this.w = fMove(this.w, tw);
this.h = fMove(this.h, th);

this.oDiv.style.left = this.x + "px";
this.oDiv.style.top = this.y + "px";
this.oImg.style.width = this.w + "px";
this.oImg.style.height = this.h + "px";

// if reducing and size/position is a match, stop the tick 
if ( !this.bExpand && (nHit == 4) )
{
this.oImg.style.visibility = "hidden";
this.oDiv.style.visibility = "hidden";
this.oThumb.style.visibility = "visible";

this.bTicks = false;
}

if ( this.bTicks )
{
var pThis = this;
window.setTimeout(function(){pThis.tick();},25);
}
}

"локально":
<img src="img.jpg" width="100px" onmouseover="this.style.width='200px'">
"удаленно":
<img src="img1.jpg" width="100px" id="img1">
<img src="img2.jpg" width="100px" onmouseover="document.getElementById('img1').style.width='200px'">

<!DOCTYPE html>
<html>
<head>
<title>temp</title>
<style type="text/css">
#block {
    background: #333;
    width: 100px;
    color: #fff;
    font-family: Segoe UI, Arial, Helvetica;
    text-shadow: 0px 0px 3px #000;
    padding: 30px;
    -webkit-transition: all 1s ease;
       -moz-transition: all 1s ease;
         -o-transition: all 1s ease;
            transition: all 1s ease;
}
#block:hover {
    width: 450px;
    background: #486;
    text-align: center;
}

#block2 {
    width: 100px;
    -webkit-transition: all 1s ease;
       -moz-transition: all 1s ease;
         -o-transition: all 1s ease;
            transition: all 1s ease;
}
#block2:hover {
    width: 450px;
}
</style>
</head>
<body>

<div style="width:300px; height:150px; text-align:center; background:#999999;">
<img id="block2" src="img.jpg">
текст
</div>

<div id="block">Наведи на меня курсор, и я растянусь, а также сменю цвет!</div>


</body>
</html>