Q: OS X El Capitan help

Advice on installation.

i have a mid 2012 13" macbook pro with 8gb Memory and ample hard drive space for the installation

i am currently running os X 10.8.5 Mountain lion - this came pre installed on the device when purchased in 2013 and i haven't yet upgraded (for many reasons i later explain)

I now wish to upgrade to Os X el capitan  - as my safari says i need to, i also need to for pages, numbers etc etc. I need the update for my phone and iPad to link up better also

which would be best for me an my device. I presume a 'clean' installation is better- but how can i get all my files back onto my mac? ( I do have an external hard drive with time machine - but i wouldn't know what to do)

I've see you can just 'update' your os with everything staying on it- i would rather do this but fear it would slow my mac down considerably (as all the OS x el capitan reviews say on the app store)

My other main concern, is iPhoto. I have iPhoto 9.4.3 (2011) version, with over 8000 pictures. Ive read in reviews that the new photo app can delete and lose these pictures: hence I've not updated yet!

So to conclude, what is the best method of updating for my macbook pro? and will 'updating' lose my pictures??

sorry of the long winded and wordy question! 

I would rather just "install upgrade" but after reading reviews