Edit in JSFiddle

$(document).ready(function()
{
    CropImageToSquare(200);
});

function CropImageToSquare(squareSize)
{
    $('img.crop').each(function(i, item)
    {
        $(item).wrap('<div class="container" />');
    });

    $('div.container').each(function(i, item)
    {
        $(item).css({'height': squareSize + 'px', 'width': squareSize+ 'px' });
    });

    $('.container img').each(function(i, item)
    {
        executeToSquare(item, squareSize);
    });        
}

function executeToSquare(imageItem, squareSize)
{
    var width = $(imageItem).width();
    var height = $(imageItem).height();
    
    var tmp_width = 0;
    var tmp_height = 0;
    
    var position_top = 0;
    var position_left = 0;
    
    if(width == height)
    {
        tmp_width = squareSize;
        tmp_height = squareSize;
        
        position_top = parseInt(0 - ((tmp_height - squareSize )/2), 10);
        position_left = parseInt(0 - ((tmp_width - squareSize )/2), 10);
    }
    else if(width > height)
    {
        tmp_width = parseInt(((width / height) * squareSize ), 10);
        tmp_height = squareSize;
                    
        position_top = parseInt(((tmp_height - squareSize )/2), 10);
        position_left = parseInt(0- ((tmp_width - squareSize )/2), 10);
    }
    else if(height > width)
    {
        tmp_width = squareSize;
        tmp_height = parseInt(((height / width) * squareSize), 10);
        
        position_top = parseInt(0 - ((tmp_height  - squareSize )/2), 10);
        position_left = parseInt(((tmp_width - squareSize )/2), 10);        
    }
    
    $(imageItem)
    .attr('width', tmp_width ).attr('height', tmp_height )
    .css({ left: position_left, top: position_top });
}

<img class="crop" src="http://dl.dropbox.com/u/26764200/Photos/1680x1050_Widescreen_Wallpaper_Summer_1920.jpg"/>
<br/>
<img class="crop" src="http://dl.dropbox.com/u/26764200/Photos/eiffel-tower-picture.jpg">
<br/>
<img class="crop" src="http://dl.dropbox.com/u/26764200/Photos/main_picture_large.jpg">
<br/>

.container     { position: relative; overflow: hidden; }
.container img { position: absolute; }