Need to replace 1000 images - Is there an easy way?

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Hi.
One of the sites I built a long time ago using C5 has decided that they would like to place a watermark on all of their images - some ebay scammers have taken to using them.
Now, I reckon I can probably batch-process the images using Photoshop or similar to put the mark on them but is there any way to;
a. Download all the images at once?
b. Upload them all back to their original locations/ names so that none of the pages using them will be disturbed other than displaying the revised images?
It's otherwise going to be a long and boring process.
Any suggestions welcome.