My workflow to post a photo to Twitter and Instagram starts with putting the final image in a synchronized collection in Lightroom Classic. On my iPad or iPhone the collection needs to have the Store Locally setting on. I then share the photo from Lightroom Mobile using my Shortcut Photo to Social Media. The Shortcut can also be used on any Image, including photos in the Photos app.
You can install a Shortcut via the links below, but first your Apple device must be setup to allow it. In Settings go to Shortcuts and turn on Allow Untrusted Shortcuts. When you click on the links to my Shortcuts a prompt appears showing the contents of the shortcut. Scroll all the way to the bottom and click on the red Add Untrusted Shortcut button. [^1]
Photo to Social Media
During the installation you are asked if you wish to watermark your photo. Local watermarking requires some additional steps on your part. If you do not wish to watermark your images, or if you have already placed a watermark, then enter No. It can be changed to the default Yes later.
A second prompt is for saving the image to the Social Media album in the Photos app [^3]. The default is Yes, but if you do not want the clutter or to burn the space, then enter No. I save the photos because they are resized and watermarked, so if something goes wrong, I can go to the folder and post to the service manually.
00 Add Watermark To Photo
During installation you are prompted for the name of file that contains your watermark. The default is Watermark.PNG. I would leave it unless you created otherwise, see Local Watermarking.
00 Resize For Social Media
Nothing to configure, this helper resizes the original to better fit Instagram (but it is not a perfect fit yet)
Select an image in Lightroom Mobile or in the Photos app, and bring up the Share sheet. Scroll until you see Photo to Social Media and select it[^2]. You will be prompted for the caption to add to your photo. The caption is copied to the Clipboard so you can paste it into the proper place in Twitter and Instagram.
Note even though one could post to Twitter from Instagram, that image is mangled by Instagram. So I prefer posting to Twitter separately, particularly for images with a wider aspect.
Here is how to setup watermarking. It is done on a copy, leaving the original image unmolested.
Create your watermark file. PNG allows for an invisible background, so use that file type. The helper shortcut expects the file to be in a very specific iCloud location. In the Files app, open the “Shortcuts” folder. Add a new folder “Watermarks”. Place your Watermark.PNG file in the folder. You can of course use another file name, but be sure to configure 00 Add Watermark To Photo with that file name.
Test Your Watermark
Here is Shortcut that you can run to verify how your watermark gets applied. You can select a photo and use this from the Share sheet, of simply run the shortcut and select a photo from the pop up. The result is placed in the Watermarked album of the Photos app. Run this independent of Photos to Social Media.
[^1]: These instructions apply to iOS 13 or iPadOS 13. Please use web for earlier installation of Shortcuts (a.k.a. Workflow).
[^2]: I have configured the Share sheet to have my shortcut appear at top of the menu, see iOS 13 documentation.
[^3]: If you get an error, you may need to create the album in Photos first