Some image links from imgbb are displayed usually, and some are not (on browsers other than Tor). An example is the following Mixero Avatar:
The avatar is located on the same hosting as the forum, it does not go through a third external service. I would say that there is something on your side.
Although everything loads normally for me, ibb made some restrictions according to the Btt forum, so I wouldn't be surprised if a similar condition applies to Altt as well.
On the forum profile, there are several options when users use Avatar.

I used a Mixero Avatar uploaded from a PC (previously, I downloaded the image first from the link provided by the campaign manager).
I need to find out what options hugeblack uses to set the avatar on his account. I assume that he used the "Specify avatar by URL" option.
In this case, the Avatar location used by the user (who uses the 'Specify avatar by URL' option) may be different from the forum hosting location. Correct me if I'm wrong.
Recently, images stored in imgbb often have problems when I use them in Bitcointalk and now in Altcoinstalks, too. At first, I thought I was the only one experiencing this, but it turns out several other users also experienced the same thing.
I'm sorry, OP. This discussion is not directly related to the Mixero service, but I also need to inform you that the images stored on imgbb listed in the OP are not visible to some users.
In this case, I agree with examplens:
As for Mixer, as well as all other services presented here, which have their own website, it is suggested that they use images hosted on their website. In that way, they will depend on themselves