Release letter as opposed to a retro-active license?

[yanks]

I have come to an agreement with an attorney that represents a "photographer" regarding use of 3 images on our website. (we thought we had permission but the entity that gave us permission apparently didn't own the images) After 4 months of going back and forth we have come to an agreement. The "release form" they sent over basically is for these 3 images and these 3 images alone. Since we screwed up on getting proper permission on these we can't be sure that their aren't others that this company may already know of or will find in the future. (the website has been completely taken down btw)
Can I get a release that is for this or any other infringement of images they own(or may purchase in the future and then come after me because they may still be indexed in Google or elsewhere?) What I don't want to do is pay them for these 3 and then get a new demand letter a week after I pay them for these 3 because they own others that we used mistakenly)
So should I ask for some kind of general release of any and all claims from the date of the agreement back in time? I basically don't want to be paying this place large sums of money for an image at a time in effect funding their eventual lawsuit against me when I run out of money or refuse to pay anymore.