When you rendered the bottle, was it the same as the screenshot you posted earlier? The one with the black background/blue bottle? If so, I believe you need to go in and change your Environment Settings to white (Scene Menu > Environment) Change the color Ambient color to black and the color below the dropdown to white. Additionally, if you're going to bring this into GIMP for post-processing, I recommeded you render it with a transparent background. To do that, go to your Render Options and click on the Output Button and click on Transparent Background.
Let me know if you need additional help.
Art