Inline image clearly should not have the onebox treatment in my opinion
but… there are a lot of complicating factors around thresholds for on-a-line-by-itself images.
For example, this image, will never get a lightbox treatment:
Even if I hardcode it down to 100x100
Personally, I think it is fine to show lightbox for any image over 200x200 that is not inline, with a site setting to adjust threshold.