Arduino drawing a bitmap on nokia 5110 lcd
The 21 and 18 are the dimensions of the image. It's not just pixels though.

The first number (21) is the number of bytes in a row of the image - that is one eighth of the width in pixels.  The width must be a multiple of 8.

The second number is the number of rows in the image - basically the height in pixels.

The two numbers, when multiplied together, must equal the number of bytes in your array.

From the documentation:


  Description
  
  Draw a bitmap at the specified x/y position (upper left corner of the bitmap). Parts of the bitmap may be outside the display boundaries. The bitmap is specified by the array bitmap. A cleared bit means: Do not draw a pixel. A set bit inside the array means: Write pixel with the current color index. For a monochrome display, the color index 0 will usually clear a pixel and the color index 1 will set a pixel. 
  
  Arguments:
  
  
  u8g : Pointer to the u8g structure (C interface only).
  x: X-position (left position of the bitmap).
  y: Y-position (upper position of the bitmap).
  cnt: Number of bytes of the bitmap in horizontal direction. The width of the bitmap is cnt*8.
  h: Height of the bitmap. 
  




It would appear your image data is complete gibberish.  This is what I decode it to be:

 ######  #     #  #     
 ###   # # ####  #    # 
  # ## #  #  #     #    
   ## ##   #  #    #    
   #      ##     #      
 #   #  #  ## #####     


        #        ####   
#   #   #    #   #   #  
  # #   ## ###    # # ##
# #  #  #  # #     # ## 
   # # #   # # #  # ### 
  ##    ##