You can manually FTP in to the folder where your attachments are located, and delete the images you want removed. But the database will still link to those (now broken) locations.
A web browser will be presented with a broken URL, but you can use .htaccess to point all broken image URLs to an "Image Not Found" image.
# Replace invalid image url with 404 image
RewriteCond %{DOCUMENT_ROOT}%{REQUEST_URI} !-f
RewriteRule \.(gif|jpe?g?|png|bmp)$ //www.EXAMPLE.com/404image.jpg [R,NC,L]This is a good procedure if you'd like to review your attachments and remove all "adult/mature" and "graphic image" pictures from your attachments directory, while not having to worry actual 404 errors given to search engines.
Your forum database will still point to that image's location for as long as the post exists. And there is a very good chance that whatever was being discussed in that post is also related to the image attached; such as screenshots, product reviews, or instructions/guides...etc
