To figure out how much VRAM an image will use, you multiply the width and height together, multiply that by 4 to get bytes, then divide that by 1024 to get kilobytes. So a 32 x 32 image would be:
(32 * 32 * 4) / 1024 = 4KB
Transparency doesn't matter when considering VRAM, as the images are stored uncompressed.
Also keep in mind that most video cards will store the image in a texture that is rounded up to the nearest power of 2 size, so for example, a sprite that is 280 x 280 pixels would be stored in a texture that is 512 x 512, taking up a much larger amount of VRAM.