To clarify the previous post, you can change your default watermark as a tile in Manage Watermarks in your Settings. Underneath Watermark in Default Watermark Settings, select Default (Tile)
if you want to change the watermarks of your already uploaded artwork, in your Image Management in settings, select the images you want to change the watermark for, then underneath Watermark, select Default (Tile). I suggest checking Force Regenerate. The watermark system is kind of finicky though, so you may have to try doing it a couple of times or change the watermarks individually rather than a batch.
If you're uploading a new image, underneath Watermark in Privacies, select Default (Tile) (in case for whatever reason, the default watermark hasn't been set to tile). Make sure to change Authorized Viewers/ Public Viewers to Watermarked, depending on your desired settings.
If you want to create a new custom watermark, go to Manage Watermarks in your Settings. Upload your watermark design, and underneath Watermark type, select Tile.
After that, you can select your new custom watermark in Default Watermark Settings.