I don't understand what a "mega pixel" is.
I'll assume you mean 400 pixel image.
For an uncompressed image (very rare to see), using RGB it would take 8 bits per pixel. That would mean 400 times 8 bits. All images usually include metadata but it is usually small. They might include things like date taken. Metadata also includes some information that the file format might need.

The most simplest image compression is .bmp (bitmap). Its a list of all the pixels. The colors are stored in a way that takes up a little bit less space. The colors are stored in the metadata. Each pixel uses one byte to reference one of the colors in the metadata (00000000 might point to 255,0,0).
There are more advanced image compression formats like png or jpeg which compresses data much more. Please let me know if you need help with these file formats.
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