Hey Andy, as another painter I know what goes into these, all of those last ones you've posted are stunning. Your graphic design ability really comes through to give you the look of your shells.
The Iron Man one is cracking, real nice details with the shading and brave of you to use white with red behind, it's come out perfectly though !
The black and turquoise one for James is so simple but incredibly striking.
And the Nike inspired one is stunning too.
Top work fella !!! Keep it up