Luciaaafs This might be a limitation of your setup, unfortunately. Screenshots are known to come out dark when using some third-party screen-capture programs, for example Nvidia tools. While the greeting card isn't a screenshot exactly, it might be similarly affected.
What would be interesting is if you had a way to test directly on the Windows machine, without the remote connection. I have no idea whether that's possible, but either way you'd have some data. If the photos looked normal, you'd know it was your connection; if they didn't, you could adjust the settings on that machine until you got what you wanted.
If that's not a possibility, please let me know whether in-game screenshots (taken with C, not an outside tool) look normal, and also the screenshots the game creates for memories. The memory feature is a resource hog, but it wouldn't do any real damage to turn it on for long enough to capture a few memories.