[Macsee]

The OP is clearly not talking about content removed by the image services, otherwise they would have been saying something about "This content has been removed due to a violation of out Terms of Service."

I beg to differ. Image companies, especially the industrialised content thieves such as Pinterest, don't have a standardised message like the one you describe. In any case, it doesn't matter what message they have as the OP wants to grab a copy of ALL images used in posts. Whether they later turn out to be just removed or illegal or whatever doesn't seem to be of concern to him. Members may be sharing photos they've taken of beautiful countryside. But they likely post photos that others have taken too. The latter is open to accusations of copyright infringement and installing a mod to facilitate such infringement could be viewed seriously.

His "claimed" intent isn't important. The legal position is ...as he may find out in a painful way some day.

If the forum owner is willing to pay for the bandwidth, the safer option is to get members to upload the images themselves (we are talking images on their hard disk, right? ). This allows him to shift the onus on to the members. Whatever the intent, grabbing an image from a third party site and continuing to show that image on your site long after the source site has taken it down (or been forced to take it down) is begging for trouble.