There shouldn't be any prablem with using these images at all  as long as you have the rights to use those images.  Hell, most the graphics used online are used without the consent of the image owners anways.

Consider all of the stock images all over the internet.  If this was an issue there would be a million people freaking out about it.

If a link is not beneficial to your site and if you are worried about whether or not it will hurt you, there is no reason you shouldn't disavow that link.  I would recommend that you are disavowing the entire domain rather than each link one by one.  It will save you time and will prevent any further weirdness from happening on that specific domain.

Disavowing links can be tedious and annoying, but the whole concept of "better safe than sorry" comes into play here hardcore.  An intern would be extremely useful in this situation. 

Trying to forecast organic growth will be extremely difficult to do.  It really depends on what they have been doing as compared to what you are planning to do for them.

Have they been conducting content marketing?

Have they been heavily involved in social?

Have they been creating local citations?

Have they... etc. etc. etc.

I'm not saying that you should pull a number out of your ass, but conservatively estimating that you will be able to drive a 20% increase to their organic traffic should be easy to hit, especially if they haven't been doing a lot of what they should have been (see above).  If you can't get their traffic up by 20% in a year, they should fire you anyways.  Anything about that will make you look like a rock star.