According to a semi-recent survey paper by Pears et al.1,
During the last four decades, many languages have been used for teaching introductory programming. The language choice is usually made locally, based on factors such as faculty preference, industry relevance, technical aspects of the language, and the availability of useful tools and materials.
On these criteria, Processing doesn't look too bad.
What can I say to you about your own preference? I would just advise thinking about what you hope to achieve with the course. As I've said elsewhere on this site, a traditional computer science program, a zero-to-job-ready boot camp, and a short-track for non-majors might have intro courses with very different objectives, and that's as it should be.
Relevance is kind of a hidden benefit here. I've never heard of any big companies writing serious enterprise projects in Processing. However, it is based on plain old Java, which remains one of the most widely used languages today. In fact, it's sort of a best-of-both worlds situation.
Pears et al. tell the story of RIT dropping Eiffel because students didn't want to learn a language that they couldn't use after graduation, but also note that one big downside of Java is that it come with a lot of syntactic baggage that gets in the way of learning actual computer science (algorithmic thinking, how data structures work, etc.). Processing strips away a bit of the syntax that Java beginners find overwrought while still being basically the popular language that they will be able to use in the real world.
Not much to say here beyond what was in the previous section. Processing is very graphics-oriented. It'll be up to you to design a curriculum that teaches CS concepts using graphics-oriented tools.
Tools and materials
There's plenty here. A solid official website at processing.org with documentation and tutorials, a relatively small but respectable community on Stack Overflow, a subreddit, and a FOSS culture that has ported Processing to other languages/platforms.
Simon Fraser University recently ran an intro course using Processing. I have no special insight there, I found it by a simple web search. But if you're interested, maybe you could reach out.
1: Pears, A., Seidman, S., Malmi, L., Mannila, L., Adams, E., Bennedsen, J., ... & Paterson, J. (2007). A survey of literature on the teaching of introductory programming. ACM SIGCSE Bulletin, 39(4), 204-223.