image borders become pixelated. Any clue?

this is how it looks the image is a png

Depending on the software creating the PNG file (Photoshop, etc.), any pixels with 0 opacity are given somewhat arbitrary color information, if any.

This results in fairly unpredictable behavior when the PNG files are read as well, potentially displaying unintended color information for pixels with 0 opacity. This is often a problem when factoring in texture filtering (Bilinear Filtering), since it blends pixel color data with neighboring pixels, including those with unintended colors.