Leon, when you go to save an image for posting, and want it BIG, make the image the size you want it eg: 1200 x 900, and save it in 'Save for web', not 'Save as'.
When you are in the 'Save for web' window, there is a slider to bring the kb size down to meet posting requirements.
Example here of a large 1500 x 1125 size but within posting limits: