Greetings,
1. I do like picture of the shell, but the background seems a little to bright and harsh. What might help is you center it, and crop it a little bit more I think this would improve it greatly.
2. Just crop the picture a little bit more so it's center of attention, IMO.
3. Seems a little dark, and is that your shadow on the left side?
I'm just a noob, but that's how I see it. =D

I like the detail in the shell in 2, and i think its placed fine. If you centered it youd cut the shadow and that would look worse imo.