i stated the issue one post above. u cannot use an old resources.jar(which to me the grinders.org mod your guys are using appears to be) with an updated with an updated jpc5_obfuscated3138.jar.
renaming the jpc5 doesnt help at all, it just makes things worse, since it makes stuff harder to identify.
what u can do is this:
a) wait for grinders.org to update their mod and play with default skin for a while.
b) update it yourself -> export all images from resources.jar and put them into a fresh downloaded one
c) find an old jpc5_obfuscated and use an outdated version
d) buy a mod from me
just browsed grinders.org and saw the last version they had was from June 14 2011. there have been at least 2-3 updates from back then till today. so whatever ur using its outdated. it should actually be an easy fix but im not really willing to do their work for them.