There is a link on the help page to get this free 3rd party browser
You can only open one listing at a time to edit/create (although you could open 10 browser windows, one in each !), and I don't recall seeing a bulk edit or replace for images. However if use use the create similar option it is pretty quick.
Few tips :-
I would also suggest you upload all images to Auctiva first via the bulk uploader using IE not FF before creating your listing, much quicker then uploading into each listing.
Size shouldn't really be more than 1024x768 otherwise too big for most buyers displays and takes ages to upload, much more efficient to prepare off line first.
It is also worth preview and error checking each listing before saving.
If listing detail is long prepare it offline and copy paste into the listing, otherwise an internet glitch could easily blow away your work, been many a victim groan and sigh on here.