I have been successful in contacting Robert Showalter, who has been gracious enough to send me the photo. Please contact me at my e-mail (written out to avoid mechanical hits, rarconti at verizon dot net) and I'll direct you to him. I am still awaiting further info from him.
My research is concerning the entirety of his life, so your synopsis of Showalter's activities in the CSA is greatly appreciated. I am in the process of trying to assemble a similar running account from everything I've dug up both online and in various source materials. I've been able as well to obtain a number of letters as well as his service record in the CSA. I suspect at this point much of what we have is duplicative, but I'd be happy to discuss it.
I have gathered quite a bit of material on his earlier life, both in California and Pennsylvania as well. I'm happy to announce that all of this is planned to become both a large temporary exhibit and a smaller permanent exhibit on Col. Showalter at Drum Barracks civil War Museum in Wilmington, CA.