The "Sources ISO" contains only source codes of the software that is normally
available. Well, I lied to you a little bit - there are also some other files
related to the way packaging works. This post has some information:
https://superuser.com/a/413651
If you want a LXDE-flavored installation media, grab a "Mini ISO". Since I
don't want you to waste time downloading the wrong one, make sure the
architecture (i686 or amd64) matches the one your CPU has.
For example, I have an old laptop with a 32-bit CPU, so I had to use the i686
version. Usually, however, amd64 is fine, most recent hardware supports it.
Please forgive me if I'm writing something that is obvious to you - I don't
know how knowledgeable you are in these technical areas.
