1. Yes, that would be preferable.
2. No need - just make sure everything you want us to see is visible (:
3. yes, but forum will size down images that are too big
4. no need, you can include your concept art, but you don't have to - seeing the model is more important. Mood board is work from other people so you can't include that anyway.
5. the file size should be much smaller - about 2.4 MB or so. You will need to save in a different format, compress a bit or make the image smaller.
Yes, I know it's not ideal, but the forum has to handle thousands of images so some limits had to be set.
Hope this helps