imgBB is probably just pulling down the image in standalone style, though, not embedded in a web page. From Sciweavers’ perspective, it would just look like a user visiting their site. Once they pulled down the image and rehosted it, there’s no further contact with the original.
Or maybe Sciweavers whitelisted image rehosting sites, since the whole idea is to reduce the load on other sites.
The instructions I posted here work without too much problem. You could still use the Sciweavers editor if you prefer that.