I think so. With multiple exposures, you have to remember that you are starting with a black canvass and adding light on to it. Once you have some light on part of it, the next exposure is not going to remove the light that was already there.
I don't know what your previous experience was with double exposures, but maybe it sounds like you have done it in darkroom working with negatives. Then it is easy to add something on a white background, but once the paper is black, you just can't add no more to it. With photoshop, you are cutting and pasting a layer OVER the existing picture. That is the main difference.
Does this make sense to you?
To upload photos, you first have to resize them using a post processing program (photoshop, paintshop pro, aperture, lightroom). If you limit the size to 1200x1200 and save jpegs, you should not have any problems.
There is only one way to learn: Keep trying.