Images for upload as attachments to the forum cannot be bigger than 400K each. The size should also be limited to 800px wide maximum to display properly.

You can download this custom software that will help you easily  re-size your images for the forum or for any other use. This Windows ap.  was written by the forum admin some years ago, is safe and is free to use by anyone. It allows basic re-sizing, some additional processing, is small and compact and easy to use.

Download  and save it on your PC, then run it (setup.exe) to install.

http://www.wildfisher.co.uk/downloads/setup.exe