Bjt astable multivibrator questions
Any type of cap is fine for this circuit - it shows electrolytics in the schematic due to them being cheaply available in large values (e.g. >10uF) Previously you could not get ceramics of this size, and they are more expensive for 10uF. 

It's more usual to place the LED on the collector, for NPN and PNP as you can see with both circuits. Placing on the emitter side is fine, but you have to take into account the raised base voltage to turn the transistor on (Vled + Vbe = Vled + ~0.7V) and also raised saturation voltage (e.g. when transistor is fully on, which will be around Vled + ~0.2V)

The duty cycle will change if you only use a pot in place of one resistor - to keep the duty cycle the same you would need a dual pot to change both 39K resistors at once to keep the values the same.