How can I get my content to have the quality it first had?

Since I 'regenerated' the thumbnails, the trouble started: any new image for the Works content appear with a bad quality.

I had sorted out the blurry thumbnails by ticking 'Original – Masonry'. However, when I resized my browser (Chrome), the thumbnail lost his quality again.

Is there a way to 'flush the cache' in WordPress by the way?

Basically, any idea where I am going wrong. My content images had no problem before.
I also changed the sizes images to '0' in General settings based on a blog I read about WordPress.

I am working on a localhost. I believe my images are large enough: thumbnail is 1120px. And the content in Works is 1880px.

When I checked the code: the thumbnails are actually 544.875px width instead of 560px (like I selected as part of the 2 column layout).

img.thumbnail-master 544.875px x 544.875

And the images in content appear at 1112px width when checking the code.

img 1112px x whatever length px

I am doing some test right now to see how the content appears; and would appreciate if you could tell me if I am doing something wrong somewhere.
Like I mentioned, the trouble started when I regenerated the thumbnails.

Thanks you,
Rosanne