This is related to Firefox, not Windows so not sure why you're asking here but whatever. To get rid of certain menu entries (context menu or main menu) within Firefox, you need to download this add-on. Deselect the entries you want to be taken out and that is all there is to it
Yes,, would like to state that the Send Image and Save Image, Set as Background context menu items are default Firefox menu items. They are programed into the Software and I do not believe there is any visible settings anywhere that will get rid of them. It appears (maybe) that you would need to hack the userchrome.css or other files to accomplish what you want.