There are many ways can be used to protect your photos and keep the visibility.
You can put the watermark to different locations of your photo. You can put the watermark at the corner. You may also put the watermark on top of the main object in the photo. This can give higher protection to your photo.
Convert to Gray-scale
You may convert your photo into gray-scale and share it instead. The gray scale format can keep visibility of the photos. If someone copy the photos, they only get the gray scale one.
Another workaround to protect your photo is using thumbnail. You may generate the thumbnail of your photos and share the thumbnails.
Put lines on top of Photo
You may put some lines to cross against the photo. It is better to use line in white or light gray color.