The image in this screenshot is being displayed at 240x135 by the
style, but the file is actually being served at its original size of 16522x6190. This is complete futzing up the scrolling on that thread list page. Images should always be resized on the server side - CSS resizing just tells the browser to cram all those pixels together into the smaller space, causing performance issues like this one, as well as bandwidth issues.
I'm not familiar with Vanilla Forums, but Drupal uses a concept called Image Styles that allows you to specify how images are resized/cropped (and more) to fit the various places in which they are to be displayed. When you hit a page like the thread index, it'll look for the resized copy of the image in question and if it doesn't exist, will create the variant from the original and cache it on the server.
Fixing this will make life easier on users, but it'll also save on hosting bandwidth costs.