First upload the JPEG image (yes, it must be a JPEG) by clicking on the "upload images" under the header "Simviation" at the top of this page and following the instructions. The name must not have any spaces in it, and must be very unique, as all images are saved according to name and will be overwritten if the name is the same.
Next insert the image by posting again and using the button directly above the ;D and >:( smileys.
You will get (img)URL(/img) with the () replaced with brackets. Change "URL" to whatever the actual address of the image is. If the image is named flipper.jpg then the URL would be
http://www.simviation.com/yabbuploads/flipper.jpgIf it doesn't work the first time, try changing .jpg to .JPG