I don't think you need a certificate for Photoshop, and I don't know of anything like that. If there is something, it's not very notable, so it's not worth it.
You probably can find stuff on Adobe website. There are several MOOCS out there that will give you a crtificate like Lynda ($$$) or Alison.com (free)
Best of luck
You know Photoshop so just create your own certificate using Photoshop. Why not since there is no official source.
Yea I didn't even know there was an official certificate either. I am sure you can find an online one for fairly inexpensive but I do like Standready's idea!
Why not just photoshop one, and design a page, for everyone to get there own there as well?
By studying, practicing and passing a Photoshop certificate examination
hmmm.. i don't have any idea and just know here if any or there is a certification for photoshop expert. For my opinion i think certification is not important. If you good with photoshop then you can make money by applying a job for photoshop. From there you can proof to your boss and colleague that you are good and expert of photo shop.