not saying that at all, mine were a different set of circumstances, where I actually purchased the image in question in good faith, but Getty came at me anyway. If you are confortable with the amount all is good. The reason why I suggested they may come back for more, is simply because most users here, don't know anything about copyright and simply grab images from google searches thinking "they are free to use", which generally leads to more than one instance. By your response , you are probably not one of those uneducated image grabbers.
Based on the fact you used the image for 6 yrs and paid 300.00, if I were in your shoes I would have likely paid it as well, but don't come at me demanding 1500.00 for an image that I purchased in good faith, and when the image is a 20.00 royalty free image that is offered on many stock sites...yeah I'm gonna fight back on this scenario everytime