Is there a difference between 杀头 and 砍头？ I'm learning about 鲁迅 and the picture he
 saw of a man being decapitated. 

My teacher would switch between 杀头 and 砍头  - is their a large difference between the two? is one more formal? Is there a difference in usage? 

Sorry for being a bit morbid - but this is helpful!
砍头 is concrete. It invokes the image of hacking with an axe or sword. When it is done with a piano wire, this word is definitely not a good choice.

砍，读kǎn，用刀斧等猛剁，用力劈：～柴。～伐。 chop；chop at；hack；hew；shear. Like chopping wood with an axe.

source: http://baike.baidu.com/view/264052.htm?fr=aladdin



杀头 is generic and abstract. It does not invoke particular image.

杀, 使失去生命. To cause living thing to lose life.

Source: http://baike.baidu.com/subview/282596/9727885.htm