Click area on sprite in canvas

I have a tutorial that does almost exactly what you need for the hit-testing. See the code here.

When you click, the code draws each shape (I use rectangles but it works beautifully with semi-transparent images) to a canvas in memory (ghostcanvas) and checks to see if the mouse pixel is on a pixel that is not-transparent.

Relevant code pasted below:

function myDown(e){
  getMouse(e);
  clear(gctx); // clear the ghost canvas from its last use

  // run through all the boxes
  var l = boxes.length;
  for (var i = l-1; i >= 0; i--) {
    // draw shape onto ghost context
    drawshape(gctx, boxes[i], 'black');

    // get image data at the mouse x,y pixel
    var imageData = gctx.getImageData(mx, my, 1, 1);
    var index = (mx + my * imageData.width) * 4;

    // if the mouse pixel exists, select and break
    if (imageData.data[3] > 0) {
      mySel = boxes[i];
      offsetx = mx - mySel.x;
      offsety = my - mySel.y;
      mySel.x = mx - offsetx;
      mySel.y = my - offsety;
      isDrag = true;
      canvas.onmousemove = myMove;
      invalidate();
      clear(gctx);
      return;
    }

  }
  // havent returned means we have selected nothing
  mySel = null;
  // clear the ghost canvas for next time
  clear(gctx);
  // invalidate because we might need the selection border to disappear
  invalidate();
}

You could have the background be transparent and check images for transparency at the clicked pixel. Here's some code from one of my game prototypes:

function getAlphaInImage(img, x, y) {
    var tmp = document.createElement("CANVAS");
    tmp.setAttribute('width', img.width);
    tmp.setAttribute('height', img.height);
    var tmpCtx = tmp.getContext('2d');
    tmpCtx.drawImage(img, 0, 0);
    var imageData = tmpCtx.getImageData(x, y, 1, 1);
    var alpha = imageData.data[3];
    tmp = null;
    imageData = null;
    return alpha;
}

Before calling this I first check if the mouseclick was within the whole image.