matplotlib - image size issue

Hi,

I am trying to create a chart using a given file as background, drawing some curves on the map, and saving an 800x600 PNG.

Code Snipet:
map = Basemap( ................)

pilImg=Image.open('mybkgmap.gif')

rgba = pil_to_array(pilImg)
map.imshow(rgba,interpolation='nearest') # showing background map
..... # Plot some paths on top of background maps
# Add some text explaining map symbols
....
(plt.gcf()).set_size_inches(8,6)
(plt.gcf()).set_dpi(100)

plt.savefig('my.png',format='PNG',bbox_inches='tight',pad_inches=0) # Obtain an 620x480 image without a border

plt.savefig('my.png',format='PNG') # Obtain an 800x600 image with a white border I don't want

Any idea on how can I get the 800x600 without the white border? Any hint will be appreciated.

Thank you