Specifying Image Dimensions

Is there a way to specify image dimensions, as per GTMetrix, I receive a low score because I do not specify my height / width of my images when they are placed.

I do not see any way in which I CAN specify these values.

I’ve tried to add the dimension codes to the plaintext editor at the bottom of the page on the back-end, but it does not work for me
http://gyazo.com/c9c3d60162da6206144747ecaf575e57

I’m looking to solve these issues:
http://gyazo.com/6f85f8794271d6333abe223f22b4c61e

According to them, as well as other websites i’ve looked into I’m supposed to add height=”xxx” width=”xxx” into the tag, but there is no tag available with the enfold theme when I’ve inserted an image via the single image element on the backend editor.

Any thoughts / ways to fix this?

Thanks