Reset/Remove all thumbnail images?

I used a plugin that was suppose to set the first image I had as the thumbnail but for some odd reason (probably due to the way the theme is setup) it didn’t and now I have a bunch of random images from within the post as my thumbnail. It looks terrible! So now I am left with nearly 1000 posts with wrong thumbnails. My question is this:

1) is there a way to just remove all the thumbnails/reset them.

this would be preferred
2) is there a plugin that can override all the featured images already present and ONLY put the images that are 125×125 or smaller as the featured image?

I would sincerely appreciate the help. Thanks everyone!