To be able to submit diagrams(and gallery images) you need to sign up as a member:
http://www.skyscraperpage.com/members/
Drawings you submit should be in transparent GIF format, and make sure they are the correct size (1 metre=1 pixel) so a 100m building would be 100 pixels tall.
Many programs can be used to draw digrams, MS Paint, Adode Photoshop, Corel Draw and even MS word!
I personally use MS paint as I find it much easier to use than the others.