Basic Animation in Image Ready

Hello! I have an incredibly silly question. How do I animate using Image Ready? This is what I've been doing: opening IR, then opening all the images I want to string together, in IR. (Does it matter what format these are in? .psd, .gif, .jpg? Or not really?) Then there's that little animation bar down the bottom. When a particular image is selected, it appears as frame 1 in the animation bar. When I want to add the next frame, I click the little box icon, but all it does is give me another copy of the first image (which is still selected) as frame 2. If I click on a new image, the animation bar loses the image I'd had previously, and only shows the current selected image. I know I'm missing something major (and probably ridiculously simple) here - someone tell me what it is, please?! *l* Thanks in advance!