Is this possible? Installing OS X Lion

Hi,

I upgraded to Mountain Lion earlier and my question is....

is it possible for me to install OS X Lion 10.7 back onto my MBP with the following.


I have a OS X Lion Install DMG ( around 4.3gb )
External 250gb HD ( 100gb is full of data ) I dont want to delete it!.


I am not very experienced and would appreciate if someone could give
me the some basics on how this can be done or if its even possible?
I do not want to lose any data on my external HD.

I just want to know if i can get clean install of Lion 10.7 back on my mac.

I do not have a flash drive.


Thankyou