It turned out some of the deeper links were missing - I clearly didn't go deep enough when using the conversion tool the first time. The problem was if I scanned too deep then it might never end but now I think I've managed to capture enough for all the forum pages to work but let me know if you notice anything missing.
There is currently a maximum files size upload limit of 2MB. The image width is then automatically resized to 400 pixels. It turns out this is actually reducing most file sizes < 300kB so we probably don't need a separate compression tool.