Actually, thinking about this more carefully, why even do this through TalkImg at all? Maybe it makes more sense to set up some dedicated hosting to serve those images without entangling them with TalkImg? I'm not sure who wants that challenge, but working on the problem in isolation will probably lower (and mostly fix) the costs, which might make it easier to find financial support.
A subdomain of talkimg could be on another webhost (that keeps the DNS responsibility in one place), say the ".m" that imgurl uses most of the time. A
€5.99/month Contabo or
double this $58.88/year will (probably) be enough, and since the amount of storage if fixed I expect hosting costs to go further down in the future.
The only reason I'm not doing this myself is because I don't want to deal with copyright takedown requests.