Thank you for fixing the broken image links. I have been also looking for this. One thing I'm unsure of, it the guide up to date? The original thread is bit old. Is there any smartphone guide available? Many doesn't have any desktop devices.
All of the software mentioned in this article should work with the latest operating systems, and last time I checked, they had the exact same user interfaces as seen in this guide. So it should still work right now.
Some smartphone apps can import your PGP key and encrypt messages with it, although I do not believe any of these have integrations with email. But it's simple enough if you're just sending text content without attachments.
On a few occasions, I have used a smartphone version app to encrypt messages and it works fine, and also has email integration and if the mate needs any further help on available help the mobile app, he can then ask to provide him with the app name.
Of course, he has to do his research about the mobile apps before using them to avoid any possible threat if there be any, but PGP is best for PCs but not everyone has access to a PC most times to that extent, the mobile app should fit in as an alternative to convey secured encrypted messages.
I would love to test it on my mobile device. Of course, all of these are solely for testing purposes. I don't intend to use it on a daily basis. So if you give me some names and references, it would be of great help.