posts Images are not responsive in mobile (too wide for mobile screen)

Hello!
I just switched my theme to Hestia, and there are post images (not the homepage or background images) on every post that does not adjust to different screen sizes. When viewed on a phone post images far too wide – much wider than the rest of the post that fits nicely.
However, a few images are okay. Those are fit with mobile screen 100%.

I tried with below codes,
img {
max-width: 100%;
}

and

img {
width: 100%;
height: auto;
}

but those codes are not working.

Any thoughts on how to fix it?

Thanks so much!

The page I need help with: [log in to see the link]