It is possible to modify preferences in flight via dev toolbar CLI but it may introduce higher overhead than loading images. Here is Python example:
from selenium import webdriver
from selenium.webdriver.common.action_chains import ActionChains, Keys
ff = webdriver.Firefox()
ff.get('http//<URL>')
ac = ActionChains(ff)
# SHIFT+F2 opens dev toolbar
ac.key_down(Keys.SHIFT).send_keys(Keys.F2).key_up(Keys.SHIFT).perform()
# command to disable images
ac.send_keys('pref set permissions.default.image 2').perform()
ac.send_keys(Keys.ENTER).perform()
# command to disable flash
ac.send_keys('pref set plugin.state.flash 0').perform()
ac.send_keys(Keys.ENTER).perform()
# disable dev toolbar
ac.key_down(Keys.SHIFT).send_keys(Keys.F2).key_up(Keys.SHIFT).perform()
ac.key_down(Keys.SHIFT).send_keys(Keys.F2).key_up(Keys.SHIFT).perform()
# reload the page to confirm there are no images or flash
ff.refresh()