If I'm not wrong, there should be a formula about to: F=(G*M1*M2)/(r^2), where F is the gravitational force that attracts the two objects (in our case the quasar or the black hole or whatever and the photons), G is the Gravitational constant (don't remember exactly the number we haven't studied this formula yet at school, given that I'm 16, it's already good that I know it, google for it, you should find it), M1 is the mass of the first object, and M2 of the second one, while r is the distance from one object to the other one.
This basically demonstrates that everything is attracted to everything else, but that G is a number so little that it would get down all of your hopes of attracting that cellphone that's ringing on your desk while you are on your bed, making it work only if you have the mass of the earth, approximatively.
Another malus math is getting on you, is that your cellphone is as little as a ray of light, a photon, that would need you to be way bigger than Earth (crushing everything around you, but those are simple details).
Now, I don't know anything about gravitational lenses, but logically speaking, they "simply" take some photons and make them turn, turning they go faster, going faster reduces the attraction to the object (not really correct, I'll focus on this later), thus they're "catapulted" against us. If you do this with two quasars and two objects you'll have that effect.
About the attraction from one object to another, the formula I wrote up there is sheer attraction, nothing more, nothing less, however the speed of the object (the less big one) matters for this.
One of Kepler's laws said that "A line joining a planet and the Sun sweeps out equal areas during equal intervals of time.".
That basically means that a object attracted to another one to the point of creating an orbit around it, goes faster when nearer and slower when farther. We can even say that 1 (the littlest object) goes faster when nearer to 2 (the biggest object) in order to not be attracted too much to it to the point it hits 2. However, when going faster and being farther from 1 it will go too fast for that distance, and then getting out of the let's-call-it-orbit-even-though-it's-not-really-an-orbit.
That's why black holes are black, because photons can't leave it and without photons we see it as black. However, it doesn't change the colors of stuff that's far from it, because the photons have an about to non-existent inertial mass and they are, indeed, far.
Given those points, aside a minumum mass 1 must have (surely it's possible to calculate it, but there are too many variables for me to calculate it), it can be as big as you want, as long as you keep it at the right distance, relatively to its mass (bigger object will be farther).
Oh well, I'll go read something on them lenses when I have some time, they look quite interesting.
