How to make images unique

Am in process of doing SEO for a 300 page site which predominantly image based. Each page consist of 6-7 images and 100 words write up. My problem is all the images are leeched from other websites and are not unique.
So what do guys think the best way to make images unique as it plays a vital factor in ranking the site.

What i am doing now is:
1) Resizing the image
2) Inserting watermark

Any suggestion would be appreciated.

Photoshop.

Looks like you have it covered.
Rescale it by a few pixels and add a watermark, although I believe watermark should be enough.
Rename file and maybe make sure file size has changed a tad.