1. Open your image in Preview
2. Go to File > Save As
3. If your image has an Alpha channel, under the image type or Format, an Alpha channel check box will be ticked.
4. Simply untick this option and then resave this image as a .png file.
1. Download Gimp (free)
2. Open your png file in Gimp
3. In the menu bar go to Layer >Transparency > Remove Alpha Channel
Alternatively, for both Mac and PC users, you can simply convert the image to a JPG and then upload the images again.