While it isn't exactly a "Remove all pictures" button, there are a couple things you can do to clear all the pictures besides removing each one individually. One option is to click on one of the images to pop up the image selector window, then click the "Reset" button at the top of this window before selecting your new images. Alternatively, you can switch to externally hosted images using the "switch to externally hosted images" link below the images area on the lister page. This will clear all your pictures, then you can click it again to switch back to Auctiva hosted images and choose new ones.
