Here's your problem:
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I'm not sure you'll get your desired result by scaling a jpeg this way. It's best practice to specify your image dimensions in pixels. I see what you're trying to do, but trust me there's a better way -- sorry, no time to make an example, but make full-height side images, take the middle section and split it in half lengthwise. Align the top half (plus left side image) to the left-top, and the bottom half (plus right side image) to the right-bottom.