Never work with photoshop,the follow are not tested.

1-to test if photoshop is installed on system you can use this kind of code with try/catch/endtry structure for any COM object:

try
loObj=createObject("photoshop.application")
if vartype (loObj)="O"
messagebox("Photoshop is installed",0+32)
endi
catch to exception
if exception.errorno=1733  &&The class definition specified in a CREATEOBJECT( ) function cannot be located.
messagebox("Photoshop not installed",16)
endi
endtry

2-if you confirm that photoshop is installed , then open your object browser (vfp menu/tools/object browser)
search for com object ("adobe photoshop...") .load it on the browser,develop tree and you see all PEM,interfaces and even interfaces classes codes...

*can for additive info read
http://www.codeproject.com/Articles/14844/Automate-Photoshop
https://forums.adobe.com/thread/359912