If you still have the original source file (project file), you could use a different, free image converter (e.g., IrfanView, XnConvert, or GIMP) to re-export the image without a watermark. This only works if the source wasn't already watermarked.
If you still have the original, un-exported project file (e.g., from AVS Photo Editor), you can try a workaround: created with a non-activated version www.avs4you.com png
Broader lessons