Hi. I have searched everywhere for this info but I could not find it.
I need to know what size the watermark image should be, and whether or not I need a different size for different sized pictures.
it can be any size you like.
currently the module does not resize the watermark image, so you will need different size watermarks if you want to pick based on the size of the image you're watermarking. note the module currently doesn't support one watermark on the full size and a different watermark on the resizes or thumbnails. (the underlying toolkits can do this, but the UI for the module doesn't have a way to make this selection)
...just don't use transparent PNG-24, it won't look good in Explorer *basically it won't look transparent.
Oh... would it be possible to implement Sleight hack to resolve this problem with png-24?
Yep, if you want you could add this CSS to your theme..