The image is the problem. Graphically it's fine, but the format is incorrect. When you export to a .png
file you are saving the colors for fully transparent pixels. Then, when you rotate the image in the Saria editor these invisible colors are blending into the visible pixels.
To prevent this issue in the future find your photo editor's setting for saving transparent pixel colors and turn it off. In GIMP these setting appear as you export a file. I've attached an image of my default settings for .png files. I turn off everything and set compression to max. (Compression affects opening time and file size but not quality.) Note: I use an older version of GIMP because I don't like the newer versions.