The number of pictures that a 4GB memory card holds is directly related to the size of the pictures, but it generally has the capacity to store 1,140 photos that are 10 megapixels. This figure is based on the fact that 1 megapixel equals about 1,000,000 pixels, that 1MB is equal to about 1,000,000 bytes and 1GB equals about 1,000MB.
There are three main factors that determine the size of a photo: the number of megapixels of the photo, the type of photo and whether the photo is uncompressed. The more megapixels a photo has, the more room it takes up. A 4GB memory card holds nearly 1,140 photos at 10 megapixels apiece but only 520 photos at 22 megapixels. Uncompressed photos are a lot larger than traditional photos, so a 4GB memory card is only able to hold around 115 pictures of 10 megapixels.
The last thing that has to be considered is the format in which the picture is saved. The most common format is JPEG. These picture types are of medium quality. A smaller size file format for pictures is GIF. These pictures are small in size because GIF file formats only support up to 256 colors.