I had the idea to create this when one of my friends created a non-interactive online collage. I decided to add a lot of power to it. I got it to the point where it works decently well in Firefox and Safari, and partially in Opera (Opera 9.5 broke it however). The IE version was always very lacking mainly due to the lack of support for the <canvas> elements. Even using excanvas there were still a number of problems. It is available online - go, have fun with it!
Overall, it has many fancy features, including support to upload PNG, GIF, JPEG, among other formats. It keeps the transparency in the image (sometimes, always with PNG-24), it allows you to click and drag the image wherever, resize it (locking proportions if needed). You can clip the image by defining a polygon around the image. You can set the overall transparency of the image (even with the mousewheel). It also maintains an undo/redo history, and provides a zoomed-in area around where the mouse currently is.
But enough of that. Go and try it yourself! Make sure you click the green checkmark to make a permanent mark on the collage.