To be honest, Daniel, I don’t know. This shouldn’t be happening because the fact that we never had this problem (assuming I’m right) in IOS before is because Apple handles this automatically in iOS.
I’m not an expert, but I’ve seen very similar problems in desktop apps like Photoshop. When you work the image in Photoshop in a different color space than the one used in the app in which the image is displayed afterwards. You see your image fine in Photoshop while you work in it, but when you display that image in the web, for instance, the colors appear different. 
I wonder, then, if something has changed in IOS in terms of how it handles color spaces. Maybe they now have exposed them to the user,  for them to have better control like in Photoshop, but they have left confusing/wrong default settings? 🤔

Maybe someone more knowledgeable than me can help you out better. 

 Yes, yes it does.
Saving as JPG means you will experience that in your gallery also. 

 So, what are you recommending? 

 There is no solution… ish

@jb55 could request the HEIC file from the OS instead of a JPEG and do its own compression with higher quality. NB also does optimizations but I do not believe that has significant impact. 

 I just don’t understand what’s changed since the last update because it wasn’t doing this before. 

 Try uploading to NB directly and compare 

 Trying to figure out what’s changed too. There is also a sub setting in iOS18 that offers the format choice “again” (from HEIC to jpg) which seems to be independent of the global format compatibility setting under the camera settings. It shows up under “options” when sharing / sending the image somewhere else. https://image.nostr.build/d7e17c207b501b6445a2b51c68358cff3e5cb9ee9a0cf5df7b4f93ba277f1b77.jpg https://image.nostr.build/4e11c41ff8a999289ca91c01c83b738817adab1ae8460dd7a7308c79ad287da1.jpg  

 Colour space, as someone else highlighted, is also a possible issue too. RGB, sRGB, AdobeRGB, monitor/LCD colour profiles, Apple colour profiles etc etc. Who knows what’s being included or stripped out upon upload. 🤷‍♂️ 

 I can no longer trust the @primal uploader on mobile since it doesn’t have previews. 

 By that I mean accurate previews, i.e., the thumbnail didn’t look like the final upload or couldn’t be enlarged to check. 

 So looks like a primal thing? Is it as bad with other clients? 

 No, I think it’s an Apple issue, I just haven’t had time to figure out what’s causing it. 

 @nostr.build @The Fishcake🐶🐾 Have you run into this issue with images uploaded from iOS 18? It only seems to happen with photos that have had level adjustments made using the photo editor. 

 Can you please elaborate what issue you are experiencing exactly? Is it color shifting, resolution decrease and blockiness, something completely different? We do convert HEIC to JPEG upon upload and it may change the colorspace and bit depth when it’s doing it. HEIC generally has higher bit depth that will make photo look more realistic but JPEG can retain most of that in sRGB space but not strong highlights or deep shadows. If you have sample pictures (can upload to your account and share with me) that would help. So far I haven’t ran into issues with my iOS 18.1beta6 

 It appears to be a color space issue. I’m having trouble reproducing it at the moment but I’ll update this post when it happens again. 

 Uploading from the account page specifically seems to remove color profile data for some reason. https://os.hel1-c1.semisol.net/static-n/color-profile-test.png 

 I’m on iOS 17 though. It works fine with HEIC also from Damus. But it most likely is a color space issue. 

 Fixed for PNGs now, had issues with pngquant being out of date and with no profile support (thanks Semi for spotting it)