So did Al-Qaida, the Taliban in Afghanistan, etc, etc. They have no real means of long-term survival, all their weapons and equipment is stolen, what happens when they need spare parts and repairs?
But I digress from my earlier point. If ISIS/ ISIL/ IS/ The Islamic State wants to succeed it needs to get its name sorted and choose a better flag. Here's something I knocked up in 2 minutes in GIMP, they insist on using the black and white colour scheme so I assume they're going for the 'pirate' look, so threw together some nice Islamic imagery in a manner vaguely reminiscent of the Jolly Roger.
You're welcome, fundamentalists. Please note, this is the 'classic' arrangement, the swords can be exchanged for kalashnikovs on request to represent a hip, modern State of terror who knows how to connect to da yoof. I'm available for comissions on the condition that nobody feels the need to scribble 'God is great' over it.