Sorry to hear about that. This add-on doesn't have a feature to delete saved images, but the images are handled as downloads and Firefox itself has a feature to delete downloaded files. The way that works is to right-click an item on the download history list and click Delete -- if multiple downloads are selected, all of the files for the selected items will be deleted. If the files weren't deleted through that feature, perhaps they are in the Windows Recycle Bin (if you run windows) or your system's equivalent. If not, you could try "unerase" software like https://www.ccleaner.com/recuva