You can make the background transparent by making it that bright green (or any colour that doesn't appear in the building itself), and then selecting "save as transparent" in the save as gif dialog box.
Typically, I I leave the background white (RGB 255 255 255), since there will be no pure white parts of the building, and I select that. That makes re-sizing easier (if you draw at a larger scale), as re-sizing with a green background will cause pixels on the edge of the building to turn greenish or pinkish. (I always used magenta, RGB 255 0 255)
A couple other tips:
make back-up copies of your diagrams before re-sizing. I often made back ups periodically while drawing so that I could go back to an earlier version if I didn't like where I was going with it.
If the proportions aren't correct, you can
resize it with different ratios for width and height. The example I used above was scaled 40% wide and 50% high. Irfanview offers multiple sampling options for re-sizing:
I am referring to the highlighted sections here; Press ctrl+r (image>resize/resample) to see this dialog
Here is another tip to make more realistic diagrams:
1. SAVE A COPY OF THE ORIGINAL BEFORE DOING THIS. It will make your diagram very difficult to alter after the fact.
2. Open the full-size diagram if you're drawing at a scale greater than 1 pixel per meter. (I often drew at 2 pixels per meter.)
3. Press "ctrl+e" (the image>effects dialog), and go to sharpen.
4. Move the slider around until you like what you see (if you go too high it looks absurd; err on the low size), and then hit apply. If you don't like it, hit ctrl+z immediately to undo it.
5. After saving the original file, re-open it in paint and paint anything you want to be transparent using RGB 0-255-0 green.
6. Follow the re-sizing to transparent gif steps above.
The sharper feature adds texture to the diagram, making it look less like 16 colour pixel art and more like an actual diagram of a real building. This works better with concrete surfaces than with glass surfaces. If you apply this technique to your London Insurance Building, it would probably look better.
Another feature is to use textures in a photo editor (like photoshop, gimp, or Paint.NET, which is what I use), but you might not be that far along yet. You can use that to make windows shine, and to add more realistic shadowing to corners in the diagram. In some of my later diagrams, I made the whole building the same colour (even though it was two faces), and simply put a transparent grey shape over the wall that wasn't lit by the sun. For night time diagrams, I used a transparent blue over the whole diagram in addition to the transparent black, and then added yellow over the windows that I wanted to light up. You can add up-lighting and other features.
You don't have to try these now, but it is something to think about for the future as you get better at diagrams.
And you will get better. I didn't think I could ever do this when I first joined the site:
My first and last. I should start drawing again.
Just keep drawing, and don't be afraid to experiment. But remember to save backups
. I am emphasizing that based on my own experience with a few diagrams.