The "problem" is simple, you have an image with the right height of a slide, the the width is not enough. In the case of a 1024 × 768 slide, some images come in the 512 × 768 size, so what to do with the other half? Case in point,
|Figure 1: Original portrait image.|
|Figure 2: turning Figure 1 into a full side slide|
1. Open the original image in the Gimp or Photoshop and resize the canvas (Image menu -> Scale Image) to have the same width of the slide. I have already done this in Figure 1.
|Figure 3. Selection of 20 pixels on the right border|
3. Open the Layer dialog (Ctrl-L)
4. Copy the selection (Ctrl-C)
5. Paste it into a new transparent layer by clicking on the left-most button on the bottom of the Layer Dialog.
6. Move to the layer you just created and stretch that selection (Layer Menu -> Scale Layer) to 532 pixels (see Figure 4). 532 = 1024 - 512 + 20. 1024 is the slide's width, 512 is the image's width, and 20 the width of the selection.
|Figure 4, Stretching the border selection|
8. Blur "step 7" (Filter Menu -> Gaussian Blur) by 10 pixels.
There you go! You have arrived at the result.