Problems transferring photos to a dig...

I have a Matsui digital photo frame which takes USB sticks and memory cards. I have an SD memory card and a Canon camera (the SD870 IS) which accepts them. I have a series of photos on my laptop which I would like to transfer to the memory card for display in the photo frame. It seems like the obvious way to do this is to upload the photos back onto the card (through the camera). In fact this has proven impossible. Surely there ought to be some way that I can transfer files back onto the camera? I don't have a separate card reader but I can't believe that I really need one for this - it seems completely counterintuitive. Alternatively should it be possible to transfer from a USB stick to the memory card while they're both inserted in the frame? Or is this too complicated for the frame software to cope with?

Sorry, I am just incredibly frustrated! Thanks for your help!