Emulator

Have you tried enabling XFB2RAM? A lot of times the game will read XFB instead of EFB for this kind of thing.

XFB2RAM is just the disabled setting for "Store XFB Copies to Texture Only" right? If that's the case, I already had that set. Enabling it did not change behavior either. The only XFB setting I have enabled is "Skip Presenting Duplicate Frames".

However, I don't think this is a graphical issue. It's not that the photo attached to the message is corrupted or something, no message seems to be generated at all.

I can also confirm that nothing is saved (with "Store XFB copies to texture only" both checked and unchecked). The game also knows that it already saved a message if you try again (even after restarting it), but I think that it's just based on keeping a timestamp of the last saved photo and not letting you save one until the next day (as traveling back in time to an earlier day won't let you save one either), and not by querying the message board and finding an invisible message, though.

Oh, I see. Sorry, I misunderstood the issue.

I did some more testing, and interestingly, saved photos still won't work if saved on console. I used bootmii/boot2 to create a NAND dump before the system menu loaded, and after importing that NAND into Dolphin, the photo isn't present. (But it did show up after I started the Wii menu from bootmii).

Another weird thing is that even though the clock seen in the system menu was WILDLY off in Dolphin (before, I didn't bother to set it, so it was some time in 2010; after importing my NAND the time ended up being in 2028), when saving in Dolphin the date on the message in shared2/wc24/mbox/wc24recv.mbx is the correct time. So, somehow, Pokémon Snap is getting the right time.

I remembered that there are other games that post directly to the message board.

I was only able to test Super Mario Galaxy, but both the photographs of the Star List and the picture obtainable from the Grand Finale star course at the end of the game posted to the message board with text and an attached photo. The Photo Channel also detected these, let me edit them, and post the edited copies to the message board which also showed up without issue. Allegedly Metroid Prime 3, Super Mario Galaxy 2, Rabbids Lab, and Kirby's Epic Yarn also all allow you to post photos to the message board from in-game. If anyone has these and can confirm if the images post on Dolphin it might be interesting to find out if the problem with message board posts is specific only to Pokemon Snap or if certain games are affected and certain games are not.

I did a bit more digging into this. As it turns out, I was completely wrong about what the problem actually was. The message board post with attached photo is generated by Pokemon Snap perfectly. However, it is stuck inside wc24recv.mbx in it's raw pseudo-email format (along with some mystery data in wc24recv.ctl that I can't decipher except for the string NAKE which is Pokemon Snap's ID). Manually decoding the text and image attachment components from base64 and then overwriting an existing message board post shows that the data is all there, it's just stuck in here instead of being moved to cdb.vff where it should be. I also learned here that there is at least 1 other game I've used with Dolphin that is affected by this: Wii Play. There's an email sitting in wc24recv.mbx about earning a gold medal in Shooting Range, but there is no such message on the actual message board. That one doesn't even have a photo attachment, it's pure text.

To summarize: The issue is not with Pokemon Snap, nor does it seem to be a date/time issue. Something about how Dolphin handles message board posts means that some emails are stuck in the temporary mailbox indefinitely and thus never post to the message board properly. But this also only happens with certain games. Pokemon Snap and Wii Play are bugged, Mario Galaxy and the Photo Channel are not.

If anyone is interested, I'm happy to upload any of the relevant mailbox or image files, though I suspect this should be easy to reproduce for anyone who owns one of the affected games.

I wonder if it's just that Dolphin doesn't support Wii Connect 24 services...

This may not be relevant information, but I tested that both Pokemon Snap and Wii Play can post to the message board on the Wii U in Wii mode on hardware. I don't know how exactly how any of this stuff is implemented, but I had heard that the Wii U didn't have Wii Connect 24 capabilities. If that's the case, it might point towards some other issue Dolphin has.

I saw in the latest (December 2022) progress report that some Wii Connect 24 features were implemented in Dolphin and booted up Pokemon Snap to check and see if that fixed this issue. Unfortunately that doesn't seem to be the case. If it is a Wii Connect 24 service, it is some other still unimplemented aspect of it.

I see that noahpistilli is working on implementing many features of Wii Mail. I'll try and keep track of when their PRs are merged and see if any of the changes affect this issue, but as of 5.0-20078, the most recent development version that seems to interact with the whole mail system, there is no change to this issue.