It really depends on the photos, the trip and if the photos are up for comment or just for looking.
Most of the time I prefer multiple threads with a few images each - it's easier to read through and I don't feel like maybe I've missed some. When I'm looking at a thread with a dive report or trip report, 10-15 images is not too many. I'm viewing these are part of the report, not to nitpick each image. Different goal. Sometimes you just want to look at/share what you saw!
I hate linking. To a gallery is fine, I guess, but I rarely will link out - I just don't have the time. Show me what you want to share and don't be stingy with the images. Add then link to the rest of the gallery for those who are interested and if you have tons and tons of images.
If constructive criticism is wanted on the images, those images need their own thread and it would be really nice to have it marked as such. Images where people are looking for feedback should be clearly numbered and each thread limited to three images - more than that and there's too much information. Concentrate on fixing the problems and trying new things first.
If you only have an image or two that you want CC on and it's part of the main dive or trip report, I say make a clear note next to the image - that way ppl understand that you think there's something not quite right in your mind and you'd like some help.
Yes, I understand that every photo could be better
, but concentrating the information works better.
The general "please tell me what I could do better" or "tell me what you think of the photos" is fine, but tends not to garner as much useful feedback. No one really knows how serious the poster is about getting critique or how detailed the poster wants. Better to have a dedicated thread. The few we've had offer some great and in-depth info.
Just my two cents.