Click here for an example. The higher the color depth that is supported by an image format, the more space the files take up on your hard drive. With digital photography color depth and resolution are the two important facets of image quality, you would rarely reduce the number of colors in an image if you wanted to retain quality. Regardless of whether you use compressed images or not, an image is expanded to its full size in memory. To work out the minimum memory you need, you must first calculate the size of your largest image using the preceding formula.
Virtual memory create a memory overflow area on disk in a swap file.
This technique enables you to run more programs at once on your PC than actually would fit in the installed memory. It works because not all of these programs want to run at once, and even for active programs they do not access all the memory locations they have allocated to them. If however you run something like a graphics editor where all the images you want to manipulate at once won't fit into memory then suddenly the PC is having to continuously swap memory back and forward from disk. In really bad situations the PC will put out a block of memory to disk and then a few milliseconds later have to retrieve this same block again. This condition is called thrashing with the PC doing little else but reading and writing memory to disk.