This worked for me as well, so thank you! For those of us that aren't particularly techy, here is how to find the block ID for the image you're wanting to prevent from stretching. Make sure it is the ID that starts with "block". Then, to enter the CSS, go to Design > Custom CSS and copy/paste the code above, replacing the "block-71dg10454aa196265419" with your block ID. You can do this with multiple images if you move to a new line in the CSS and repeat the process for each block ID.