If you are going to upload it to the request thread, post it no larger than 1:1 scale (the size it will be when put into the diagrams) and put a watermark on it (your screen name will do, partially over-top the building's diagram but not completely covering it, so that no one can steal it.) Make sure you make a copy of the diagram to watermark, don't watermark the original and accidentally destroy it.
To host it, I suggest
http://www.imageshack.us. After uploading the image, copy (highlight and ctrl+c) the topmost link (in the orange/yellow highlighted area, big text). Go to the thread creation page, hit the

button, press ctrl+v to paste it into that box, and hit enter. The image will be posted with the request.
To upload a diagram in the diagram system, you have to be logged in. Find the building's information page, in the top left of that page is "upload diagram", click on that and follow the instructions. Diagrams for the database don't need watermarks.
Make sure the image is the proper scale, where 1 pixel is 1 meter. A 29 meter high building should have a 29 pixel high diagram.