Introduction:

MIUI 10 on MI 9 gives us the ability to “slightly” personalize the Camera Watermark used by stock MIUI Camera, which is stamped on the lower-left corner of each photo when the feature is enabled.
This customization is quite limited since it allows:
- One line of text no longer than 18 letters- It won’t allow to change nor hide the first line of the watermark (“Shot on MI 9”- It won’t allow changing the watermark logo
I am well aware there are thousands of third-party applications on PlayStore which will allow editing of the photos, using whatever image as watermark.However, in my personal experience as a photographer, there are some circumstances where “fast is better” and you do not want to take all the extra steps to watermark a photo intended for quick social consumption.
This is a “dirt” way to overcome the limitations mentioned above while retaining the ease and speed of implementation that I believe can interest a large part of users.
DISCLAIMER:By following this tutorial, you are going to alter system files: although it is not a “critical” MOD, I am not responsible for any kind of trouble this may cause.As always, backup is better than restoring. Make a backup copy of the original watermark before doing anything.
Requirements:You will need a Rooted device since the watermark file is part of system filesYou will need a File Explorer of your choice installed on your device
Procedure:Open the MIUI Camera, enter the Options menu, and if you haven’t done so already enable the Watermark optionSelect the “Custom Watermark” from the same menu, and enter some text.Confirm and save by clicking on the check icon in the top right corner of the screenForce close the MIUI Camera and Open the preferred File ManagerThe original custom watermark file is called “Cepheus_custom_watermark” and it’s stored in /data/data/com.android.camera/files/cepheus_custom_watermark.pngMake a backup copy of this file, then export to your preferred graphics editor for customization.Default watermark is 1800x195 px @72ppi. I suggest keeping the same specifics for the new file. Remember to export the new watermark as PNG with Transparent Background.Once your new watermark is ready, you simply need to copy the PNG file to your device, overwriting the original.Restart the MIUI Camera and voilà, your watermark is now available.
Update for Latest MIUI 11:
With the arrival of MIUI 11 some details are changed regarding the stock Camera app and the “workaround” to change the default watermark.
One major change is that there are now two watermark files:cepheus_main_space_custom_watermark.pngcepheus_main_space_ultra_pixel_custom_watermark.pn g
As the name suggests, the second file is used when taking photos using the "48 MP" option.
The second less obvious but quite obnoxious change regards the use of a "remote download" function to replace the original watermark file if tampered.
It took me a bit of trial-and-error to understand that the camera application was actually downloading the default watermark file each time the application was closed and/or the phone was rebooted.
(this is why you might have noticed a strange behaviour where your watermark is initially changed without any issue but then appears to revert back to the original)
In order to solve this issue, you need to prevent the MIUI Camera Application from accessing the internet.
You can do this in any way you like, I prefer using NetGuard by Marcel Bokhorst. (link)
If you use the same method, here are the two process that needs to be blocked:com.xiaomi.cameratoolscom.android.camera
Please Note: If you use the "Short Movie" advanced animations feature I advise to download all the required files before customizing the watermark, thus blocking camera application from internet access.

Final Observations:
Editing again the watermark using the “Custom Watermark” option inside MIUI Camera options will probably cause some issues. If you want to revert to the Original Watermark either restore the backup *.png or restore MIUI Camera Data.
Future ROM Updates will, quite certainly, restore the default watermark: keep a copy of your personalized watermark somewhere on your “data” partition so you can easily restore.
Future ROM Updates will not overwrite the custom watermark files, unless you perform a "Full Wipe" or you perform a "Data Wipe" for the Camera App. Anyway, since it's always a good practice to keep a backup of your files, I strongly suggest keeping a copy of your custom watermarks someplace safe.