Are you using v7 or earlier? I don't think tpcmd works with v8 and newer.
Tpcmd.exe probably has to reside in the folder where ThumbsPlus itself is installed, and may have to be executed from that same folder. Also, I'd use a short folder name for the folder that holds the images, eg. C:\SLIDES.
Alternatively, you could use Image | Build Slideshow in v7 to make a self-contained slideshow in .exe format that might suit your purposes.