800 Byte Header on Blobs ???

[el_supremo]

That's the GIF header. The blob looks the same as if an entire GIF image, including the header, had been read into memory.
For a description of the header see, for example:
http://ptolemy.eecs.berkeley.edu/eecs20 ... s/gif.html
768 bytes of that 800 bytes is the colour table - 256 colours x 3 bytes per colour

Pete