In the true sense of the purpose of this particular subreddit, you're essentially correct. Although, that being said, Dinitrogen Monoxide is believed to induce an opioid peptide release in one's central nervous system upon inhalation. In case you weren't previously aware, opioid peptides are essentially what opioids and opiates mimic in one's body.

So, as it says, "This is a place for pill porn, dope porn, and all assorted things related to opiates. Anything is welcome here, within reason!" in the sub's description, OP isn't necessarily posting anything that goes against what the subreddit was created for. So yes, although the connection between Dinitrogen Monoxide and our beloved (M)U-Opioid receptor (partial) agonists, oftentimes of the Opiate Analgesic class of Psychoactive substances, is quite weak, OP hasn't done anything wrong, strictly speaking. But, not strictly speaking, you're correct, /u/LowkeyOpee. OP posting a picture of their Nitrous Oxide canisters would be equivalent to anyone else posting a picture of their DXM-containing antitussive preparation, or Salvia Divinorum smoking blend; although they're technically Opioids, in a sense, they aren't Opiate analgesics, which is what this subreddit is primarily concerning.

Anyway, I don't particularly agree with your comment regarding Nitrous' synergy with opiate analgesics, /u/White_smoke2. I've combined Nitrous with a vast range of Psychoactive substances, and the only thing that I've really found them to have considerable synergy with is Psychedelics, as a whole. I love the ever-living-fuck out of Nitrous Oxide, and I've spent many thousands of dollars on supporting my use of the substance, so seeing this particular photo on this subreddit was a (not necessarily welcomed, but more interesting) change, but you may want to consider posting this elsewhere (namely, /r/Nitrous, or /r/NitrousOxide), /u/White_smoke2.