5 STAR IMAGE TARGET NOT RECOGNIZED

Hi, I created an Image Target which is 5 stars augmentable, but it doesn't work at all. I tried to remove some tracking features by erasing a parts of image, it works better, the image s recognized, but when I try to save this new image as PNG to get transparent the erased part of image( so I can use this new Image Target to recognized orginal image), I'm unable to upload it in the Target Manager. The response is: "only 8 bits or 24 bits permitted." I saved the PNG in Photoshop/ Save for web/ 8 bits. Windows / File properties/ Details shows it as 8 bits format, but I'm still unable to upload it in Target Manager. I also tried to use an on-line png converters, but even the converted images get the same error response from Target Manager. Can anyone help me, please?