Based on my experience as a non developer/engineer hobbyist photographer getting into FOSS photography:
Not only are most people not aware of FOSS tools, when they do hear them it is usually in the context of a question posted by someone on the lines of ‘Can I use GIMP instead of Photoshop’ (or other open source tool and its equivalent) and the reply usually lists all the things you can do in Photoshop but not in GIMP. They never hear of all the things that you can do in GIMP but not in Photoshop. This encourages the notion that OSS tools are toys for nerds to play with rather than something that grown men take seriously for work. Talking of specific issues faced by photographers that are solved by OSS but not by proprietary software may help with this.
An obvious example I can think of is Magic Lantern (or CHDK) which is one of the OSS tools that many photographers have heard of, and does not have any challenger in the proprietary world.
It may also help to emphasise that many FOSS tools are cross platform and easily available on Windows and Mac, and users don’t need to use Linux to get started. Many people might be willing to download and experiment with an individual tool but will draw the line at getting to grips with an entirely different OS just to try out a Photoshop alternative.
Another point is ease of getting started with the tools themselves. I was (and continue to be) pleasantly surprised at how I can download, install, and use an OSS tool without needing a single login or signup anywhere. There are no hoops to jump through and you don’t have the uncomfortable feeling that you’ve revealed so much personal identification information about yourself that even social media companies would be impressed. And you don’t need to be paranoid about cloud based solutions.
Pixls.us is also addressing another issue that was faced by OSS beginners: Previously, if I faced issues with an image stack I might have had to register on the RawTherapee, GIMP, and Hugin forums before I solved it - and this is without considering the frustration of never being able to register on the RawTherapee forum. Now we have a one stop shop where we can get help for all these in a single thread from users, which is very similar to how Photoshop forums operate.