Scripts without picture are less fun. My advice is to look for pictures galleries, images have often the good size, great quality, and it's not too much difficult to find exactly the good one. Drawbacks : pictures too much hardcore (naked after the 2nd picture...), or not enough natural. Generally it takes 30min to find a full good gallery for a script.

Lets post here the most useful galleries aggregators :

http://badgirlsblog.com/
http://www.nngalleries.com/
http://www.babefinder.net/
http://www.glam0ur.com/
http://www.brdteengal.com/
http://www.imagefap.com/
http://www.hotyoungpussy.org/
http://www.thehun.net/
http://www.stocking-tease.com/
http://www.baberoad.com/

Also, most if not all of them work with the gallery import tool of SexScripts (go to the page, copy-paste the address into the import box, wait, add text).

Which size ? As big as possible and the best quality possible, but not too much bytes ! As it is impossible, here is an example setup : 1000px by 800px in JPEG with a 75% quality (that is around 100kb) (so use Photoshop, Gimp etc. if you know how to and you are far from this). Picture will be sized down if needed, but never up.

Great stuff doti, thanks a lot!
I had some trouble finding galleries beforehand, but now I've found a few I can use and will do my best to.

edit;
Uh oh, now I've gone in to overload. So many great sets around here, and I've only checked out two sites. nngallery, and one I was lead to through one of them, http://www.babesandgirls.com (not on the list) which has been an absolute gold mine.

Another webpage that can be very useful is http://www.urlgalleries.net/

There you can search for galleries and if you're looking for some model in particular you can search her by her name.

I find this site very useful:
https://girlsreleased.com/

Do the copyrights of the pictures on the linked sites allow using them?
Or do I risk copyright issues when I use them in a script?

Is this illegal ? Yes, in most juridictions (IANAL)
Is there a risk ? Not much, especially due to the anonymous nature of SexScripts ; for example, the removal form on the scripts page has not received any cease and desist order ever