EWWW optimzes "dummy" images

Hi, we had a problem with bulk optimization hanging after 50-100 images, So after trying a server priority optimize I just opted to reload the bulk optimize page and clicking resume about 100 times until it did about two thirds of our media library, and now it does a strange thing: when I click resume, it optimizes these strange images listed below – some are reapeating- and every resume does the same set exactly (even though we still have a few hundred images left to optimize in our media folders)

What are those dummy images? and why does dummy/img5.jpg and others appear twice there?

thanks

Optimized image: dummy/img1.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img4.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img5.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img6.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img7.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img4.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img2.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img1.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img3.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img5.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img6.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img7.jpg
Elapsed: 0.003 seconds

Optimized image: dummy/img11.jpg
Elapsed: 0.003 seconds

https://wordpress.org/plugins/ewww-image-optimizer/