Before you start removing the background on your image you may want to doublecheck or set a few other options in the GIMP. When you draw or paint on your mask you'll want to draw or paint with the foreground color set to black.
Your background is probably white at this point. Since your frame is also white, you may find it helpful to switch to the background layer and fill the background with another solid color that constrasts with both your frame and the main subject of your photo. Gray, red, blue -- it doesn't matter as long as it is solid. You can change the background later. When you begin the next step, the background color is going to show through and it's helpful if it is not a color that blends with your frame and photo subject.
