Because copies of your HTML files and images are stored in cache, it is impossible to prevent someone from being able to save them onto their hard drive. If you are concerned about your images, you may wish to embed a watermark with your information into the image. Consult your image editing program's help file for more details.
The colors on my page look different when viewed on a Mac and a PC.
The Mac and the PC use slightly different color palettes. There is a 216 "browser safe" color palette that both platforms support; the Microsoft color picker page has some good information and links to other resources about this. In addition, the two platforms use different gamma (brightness) values, so a graphic that looks fine on the Mac may look too dark on the PC. The only way to address this problem is to tweak the brightness of your image so that it looks acceptable on both platforms