Thank You, both of them are working as I wanted.

Please add this one as well: 

#wpforo #wpforo-wrap .wpforo-post .wpforo-post-content iframe {
    margin: 0 auto !important;
}

But the CSS for 2nd link is forcing the width of all the images to 100%, which is not what I want. Can't we just force the justification to center ... ??

Use this one: 

#wpforo #wpforo-wrap .wpforo-post .wpforo-post-content img, #wpforo #wpforo-wrap .wpforo-revision-body img {
     display: inline-block !important;
}

Youtube embed in the first link is exactly what I wanted, So first part of CSS is working fine, but it'll only work for youtube embeds or others as well...?? I'm asking because its not working for instagram links, instagram embeds on this page are not centered.

https://yatrahelpline.com/community/photography-club/international-monuments/#post-83

But the CSS for 2nd link is forcing the width of all the images to 100%, which is not what I want. Can't we just force the justification to center ... ??

Try this CSS code:

#wpforo #wpforo-wrap .wpf-embed {
     margin: 0 auto!important;
}
#wpforo #wpforo-wrap .wpforo-post .wpforo-post-content img, #wpforo #wpforo-wrap .wpforo-revision-body img{
    width: 100%;
}

Insert the code in the Custom CSS code textarea located in the following tab: 

For the Single Board Forum:

• Dashboard > wpForo > Settings > Colors & Styles

Multi-Boards Forum:

• Dashboard > > Settings > Colors & Styles tab
• Dashboard > > Settings > Colors & Styles tab

For example:

• Dashboard > EN Forum > Settings > Colors & Styles tab
• Dashboard > FR Forum > Settings > Colors & Styles tab

https://yatrahelpline.com/community/main-forum/parashar-dham/#post-59

https://yatrahelpline.com/community/around-the-world/moscow-top-10/#post-48

At the end of the page on 2nd link, there is a image which is Center Aligned, but the first picture is not, even though both have same settings.

Please leave the example URL to allow us to check the issue.