Thumbnail sharpness in WooCommerce, responsive layout

Dear Kriesi Team,
I’ve got a question concerning the enfold theme in combination with the WooCommerce shop plug in. We are a company, that would like to sell video clips via the woocommerce shop. The whole layout incl. programming was done by a programmer and is oriented on big video agencies like shutterstock.com or corbismotion. We created a catalog of small thumbnails (jpeg) that are loaded first. While mouse over, a low res video starts to play. By clicking on the video, you get into the article details and can buy the article. Here is a screenshot of the catalog view incl. mouse over effect:

WooCommerce Thumbnails" />

We have noticed, that all small thumbnail images are a little bit unsharp, although the woocommerce thumbnails size (Woocommerce settings -> Image options) matches exactely the pixel size of our source jpegs, that I’ve uploaded to each article and which are clearly sharp.
Woocommerce or WordPress must either do an unnecessary resizing, that we don’t want or must apply a compression to the jpegs.
Due to the responsive design, the images getting more and more unsharp when reducing the windows size of the browser. I found out, that woocommerce scales the thumbnails to five different pixel sizes, depending on the size of the browser window.

Thumbnail size full screen window 187×105
reduce window size Thumbnail jumps to 159×89
reduce window size more Thumbnail jumps to162x91
reduce window size more Thumbnail jumps to 208×117
reduce window size more Thumbnail jumps to 266×150 (its a view like on an i-phone display)

Is there a way that we can make our thumbnail images sharper throughout the whole responsive layout and avoid unnecessary image processing by woocommerce/wordpress?

Thanks for your answer!