dwwojcik hit the nail on the head I think. You can open up the texture files in the specific mod subfolder with winzip or something similar. Paint.net is also pretty neat, though photoshop takes the cake.
C:UsersyournamehereAppDataRoaming.technictekkitmainmodsMineFactoryReloaded-2.5.4-665.jar
the file you want is like 3 folders down after you open up "textures"