Best Heighth and Width for Images viewed on mobile devices

Question: What is the best height and width for home page images viewed on a mobile device? 
West Austin II Toastmasters Club 6739
(I am a former webmaster who used to know this years ago, and I am back again.) I searched the forum and the documentation and found a link describing how to change the aspect ratio or whatever it's called, but that doesn't answer my question. I have tried guessing, and it still appears I need to edit. I do not have an IT background so keep it simple.) Thanks! 

ETA:  westaustin2.toastmastersclubs.org/

Set the width to a percentage, leave the height blank (nothing in the box).

See the "Resizing and positioning images' section in the following document;

support.toastmastersclubs.org/doc/item/a...editing-page-content

Thank you for your prompt response, especially on a Sunday. I couldn't get to it until today, but I was successful. Perhaps the specific percentage could be added to the knowledge base. After trying smaller percentages, I used 200%, which seems to work for me. 

We appreciate the feedback and suggestion. Specific % depends on the page appearance a club wants to achieve. That's trial and error so we wouldn't be able to put it in a doc.

100% tells the image to display at full available width. (page, table cell, etc)