[@robkk Moderator 12 years, 3 months ago]

not sure but you probably have to set a max width for oembed embeds

http://www.wpbeginner.com/wp-themes/how-to-set-oembed-max-width-in-wordpress-3-5-with-content_width/

add the code to your functions.php in your child theme.

change 600 to whatever number you want as the width, im sure you could copy the function to width to height and also do the height to.

if the code doesnt work , look at the comments and try that user posted code. (i havent tried any of these codes so i dont know if they work really, if they dont just delete the function from your child functions.php)

for images (using img quicktag) i think you can use css

#bbpress-forums div.bbp-reply-content img, #bbpress-forums div.bbp-topic-content img {
max-width: 100%;
height: auto;
}

for images (posted by link using oembed , imgur , flickr picture, instagram picture) the oembed code that i linked to should do the same for these.