The number of pictures any size of memory card can hold, whether it be a 128MB, 512MB (Megabyte, denoting one million bytes of information), or 1GB, 2GB, 4GB, 8GB, 16GB or larger (Gigabyte, denoting one billion bytes of information), purely depends on the resolution of the images you are taking. This resolution will then dictate how many photos can fit on the size of the memory card you are using.
What is the "resolution" of an image?The resolution of an image or picture describes the amount of detail contained within the photo. A higher resolution means that the image has a greater amount of detail. In basic terms, this means the difference between seeing a fuzzy object, and seeing an object very clearly. Specifically with digital images, resolution is defined by the number of pixels in a given space, with a pixel being the smallest possible bit of definable detail in an image. They are tiny squares of individual color, that make up an image.
The resolution, secondarily, depends on how many megapixels your camera is assigned. The more megapixels, the greater the maximum image quality your photos will be.
So what are the different levels of resolution on cameras?There are currently 6 different levels of resolution. Sometimes they are allocated symbols on your camera, to represent the increasing resolution quality. These are 256x256 (very basic quality), 640x480 (low end resolution), 1216x912 (a standard 1 Megapixel camera), 1600x1200 (2 Megapixel camera), 2240x1680 (4 Megapixels, which is the current standard resolution found on most commercial cameras), and finally 4062x2704 (a really good camera, with around 11.1 Megapixels).