Open them in photoshop or some such and save the as jpg's after you resize them as a power of two, 256 512 1024 2048 ..., as the poower of too is easy for the computer to process. Also back up the originals in order to preserve them for future use. Any filitring will bring you into detailed optimization and most likely is not worth our time as you are already into your project and would require further study in order to understand the mechanics. Making smaller textures in a power of two as a jpg will decrease cpu use considerably when talking about a profesional high end model. Mind that they will no longer look as good.