Monochrome - 1 bit per pixel. So, 500kilobits is 500,000 pixels.
True Greyscale - 8 bits per pixel, and 256 shades of grey. so 500kb = 500,000/8 = 62,500 pixels
8 bit colour or 256 colours -- 62,500 pixels.
"Hi Colour," (aka 16bit colour, uses 16 bits per pixel) -- 31,250 pixels
True Colour (aka 24bit colour. Usually uses 32 bits, as it was found that it gave higher performance on 2D graphics -- image is divided into 3 monochrome layers, with 8bits for each of the Red, Green, and Blue layers respectively).
On an older video card, that actually used 24 bits per pixel, 500,000bits is 20,833 pixels with a little extra memory left over because you can't really have a fraction of a pixel (well, you can, but they're not real pixels, just a processing abstraction).
Later graphics use 32 bits for true colour (the extra 8 bits is discarded for 2D graphics, or used for z-buffer in 3D textures). Then you're down to 15,625 pixels.
It really depends on format. 500kb is just a size.
If the image was not compressed and its was stored as 8bit RGB you could just divide the file size by 3.
However you are better off looking at the properties of the image and not the size.
It depends on... colors (black+white, greyscale, 8bit color, etc.)
quality ( # of megapixels [major part of this])
and probably a bunch of other ****... so there no way to tell 4 sure
p.s. if your using a windows comp. all you have to do is right click the file go down to properties and it will tell you the size on there.
