RAM (memory) is what the brains of the machine (the processor) uses temporality to work on things, it's not permanent.
I think your referring to storage space, which is on your permanent boot drive and holds your operating system, programs and files.
So you want to make a backup to a external drive, there are various options.
1: Entire drive backup of everything (TimeMachine or Carbon Copy Cloner)
2: Just users files (storage drive)
The easiest is just connecting a blank new external drive larger than the total space on your boot drive and when the pop up window asks to make a TimeMachine drive, say yes and allow it to work (for hours and hours)
Slightly more difficult is Carbon Copy Cloner, however it's faster and the drive is bootable (advanced) it's also payware thus not free. (also the disk must first be formatted GUID OS X Extended Journaled in Disk Utility)
If you want to backup just your pictures, you can drag and drop the Pictures folder onto a external storage drive (do not use TimeMachine if it asks) and it will make a copy. This is the fastest method as your avoiding everything else.