What often happens is that PNG files have transparent areas in an undesired color, say white with alpha = 0. By themselves, all pixels with alpha = 0 naturally look the same, they're invisible.
When your image gets shrunk during rendering, though, your brown pixels will be "merged" with adjacent transparent grey pixels. The result has an alpha value that is inbetween solid and transparent, and the color will be a mix of brown and white.
If the image is not rotated or stretched and drawn at integer positions, this does not cause problems (see lol_o2's response). But the real solution is to make sure your transparent pixels have the right color. I'm not sure if other editors are better, but it's a PITA in Photoshop for complex images.
A hacky solution is often to just fill the transparent areas with #ff00ff and let Gosu load the image as BMP. Gosu replaces #ff00ff pixels in BMP files with the average color of adjacent pixels (except with alpha = 0), which looks great when stretching and rotating.