The size of an image contained within the preview pane of a Composer window may vary depending on the configuration of the browser displaying the Composer (e.g., dimension of browser window, zoom level, etc.).
Presently, the dimensions of the images from the browser creating the post containing the Onebox will be used to render the Onebox to anyone who views the post. It is safer to let the backend figure out the dimensions of the images. Therefore, exclude
.onebox.allowlistedgeneric images from the list of
image_sizes sent to the backend.