I used a share ware tool to generate the thumbnails, I haven't paid so it puts the watermark on it. . . . if it is ok I could actually pay to get the watermark "feature" disabled.
There is a program called Multiple Image Resizer .NET that is good at resizing images in bulk. You can even generate images as small as thumbnails. It will not add unwanted watermarks. You can add your own text to images, but not watermarks, add borders, crop, compress, rename, and so on.