Page Speed Specify image dimensions (icons)

Hello, I am using Page Speed and it says that:

Specify image dimensions
The following image(s) are missing width and/or height attributes.

…/plugins/news-bar/assets/images/icons/facebook.png?8addaf (Dimensions: 16 x 16)
…/plugins/news-bar/assets/images/icons/twitter-1.png?8addaf (Dimensions: 16 x 16)
…/plugins/news-bar/assets/images/icons/youtube.png?8addaf (Dimensions: 16 x 16)

I really need to change that as it says that this is a high priority (my score for this is 0…)

I will never change my icon design so, where can I specify the height(20px) and width(20px) ?

Thank you a lot by the way for this awesome plugin!
keep up the good work ??

Dorem

PS: sorry for my bad english ;o

https://www.ads-software.com/extend/plugins/news-bar/