Not quite ?, You want the tip to be at an angle to the front of the cab/speaker. In the pic you had the 57 aimed directly at (close to) the centre of the speaker cone. Just point the tip left toward the outside edge (so the mic diaphragm is (off axis) at an angle to the front of the cab). On axis is directly in line with the voice coil (pointed straight at the speaker (BTW you can try staying on axis and just moving it left or right to point at different parts of the cone (remember tops are in the centre of the cone, bass on the outer edges...)), a little off axis tilt works better though IMHO. You'd do the same with a snare drum i.e. mic it at an angle, the closer to 90 degrees, the more proximity effect but much harder for the diaphragm.
Hey El, I don't know if you already know this stuff so I'll just blurt it out:
Mics and speakers are transducers, mics convert sound waves to electrical signals, speakers turn electrical signals into sound waves. There are various ways to do this but each mic type (dynamic, condenser, ribbon, piezo etc) has an equivalent (almost opposite job but they work the same way) in speakers.
The 57 is a dynamic mic and works in the same way as your guitar speakers. The mic has a small diaphragm (mylar ?) attached to a short tube which is surrounded by lots of insulated copper coils, the tube/coils sits in a magnetic field, as the diaphragm vibrates, the coil movement generates a small voltage, a dynamic speaker (like a guitar speaker) does the same thing in reverse, it has a bigger coil/tube and bigger diaphragm (cone) and takes the amp output signal into its coil which makes it vibrate.
A condenser mic works a little differently (but does the same job (albeit much more acurately)), a condenser mic also has a (mylar?, they vary) diaphragm but impregnated with carbon (so is electrically conductive), this sits between 2 electrically charged plates (so can't move as far as dynamic without bashing (bottoming out)). When the diaphragm vibrates, it changes the capacitance between the two plates which is the signal you want, this is very low voltage so needs a pre pre amp (if you will)) to bring it up to decent level, so condenser mics need power (either battery or phantom (48v) power) to work (power for the 2 charged plates and power for the pre pre amp (best ones use a tube for pre pre amp)). Condensers are much more sensitive (you'll pick up the traffic outside etc) so tend to be used in studios where you have sound isolation and want very accurate sound. The speaker equivilent is like my QUAD ESL 63's, they are electrostatic speakers, they work the same way as a condenser mic in reverse, they are larger carbon impreganted mylar diaphragms between 2 charged plates, very accurate, but don't go too loud (as they'll hit the plates while vibrating).