If someone posted a photo with an individual with crocodiles, who happened to be wearing Crocs™, and it was hashtagged "#crocs", it's possible they did not intend to consent to be used for Crocs™ advertising material, but your proposal would assume they are. It may be contrived, but I don't think it's so contrived as to be automatically dismissed as a problem with your position. It goes to the general problem of ambiguity in hashtags.[0]
IANAL, but AFAIK having a trademark (in what jurisdiction?) does not confer the ability to appropriate others' IP even if they used your trademark. At best Crocs, Inc. can probably censor uses of the #crocs hashtag.
If she had snail-mailed Crocs, Inc. her photo with nothing else but the caption she wrote on Instagram, it would not give them a license to copy it! I feel like a broken record at this point... I agree it would be bizarre, and some people's social media behavior like this case is bizarre, but it would not justify them reproducing it without her permission.
They reproduced it on their own website, which is different than retweeting or whatever analog Instagram might have. Honestly I'm not sure exactly where I draw the line, especially given I don't know all the nuances of social media's features and etiquette, but rehosting crosses the line.
(I don't typically defend IP law as it exists today, but as long as it remains the law, I will when rich corporations try to trample lowly individuals' IP rights.)
[0] Edit: In the spirit of IP attribution, I will note I did read another commenter's post here about crocodiles and have probably derived this scenario from it. :)