welcome to the Matterhorn list! As there are a number of them, please allow me to point you towards [hidden email], this list (community@) is more targeted at audio/video/lecture capture and surroundings in general.
Now to the details: the image you are looking at is being produced as part of your matterhorn workflow, it is most likely the "image" operation that takes the name of an encoding profile as its argument. If you go to /etc/matterhorn/encoding directory, that profile is defined in one of the definition files. Now look a the commandline and make sure that instead of a fixed size (e. g "-s 300x300") it includes something that will maintain the aspect ratio "-vf "scale=400:-1" (this will create an image of 400px width and the corresponding value in height).
We are using the Matterhorn image viewer in our internal collaboration tool based on Attlasian Confluence.
But the intial still image in the video is distorted (see screenshot). When the video is playing, the image is fine. It is only the initial still image that is the problem. The code we use to embed the video is the following: