Can I root my Nexus 5 without a custom image? Is it possible ?

So far I've found 2 images that would allow me to get root, either cloudfire or team win recovery project (twrp).

I'm a little uneasy loading an unknown bootloader to my device.

Or maybe some information beyond "run this program", "double click here" and such. I work as an embedded engineer so I like to know what I'm doing

EDIT :

Usually embedded devices have a 3 stage loading. First they execute hard code in ROM when power comes up. Then this code loads the boot loader, then this boot loader brings the actual linux kernel up. It is my understanding that I will be overwriting the boot loader with twrp. By image I mean a fully compiled binary that will overwrite what is already present. If it is not an "image" to you then fine, it is not. I want to know if it is possible to root the device without overwriting the default bootloader.
29 January 2015: TWRP can no longer install official updates past 5.0 on Nexus devices. Updating my language here to say "install unofficial updates" instead of "official".



Since you specifically mention TWRP in your question, I'll just use it as my example. Note that CWM or any other possible Android Recovery project will also apply.

It looks like there's a slight misconception at play. I'll just drop a few clarifying notes here:


TWRP is indeed an image to be flashed.
Flashing TWRP does not overwrite your bootloader.
TWRP is not a bootloader replacement - it is Recovery software. It is a drop-in replacement for Android's stock open-source Recovery Mode, and therefore has the access needed to do things like


Create/Restore backups
Root devices
Install unofficial (and some official) updates



So the short answer is: Yes! It is indeed possible to root without overwriting the stock bootloader!

A few more notes:


TWRP (And CWM Recovery) is a mature (Several years old and quite stable) Open-source project that is used by a huge portion of the Android community and considered very trustworthy.
Flashing TWRP will replace the contents of /recovery/.
You generally need to use the stock bootloader in order to select the boot-to-recovery option in order to use TWRP.