I agree @skiesandsports to every aspect of this post. I have done almost everything you are writing about, and I can say that indeed you can have a nice passive income. As for stock image agencies, you have to read carefully their needs in photos, this is more profitable. Furthermore images with people have to be accompanied with the “model release”. Very important.
By my experience, I had more profit when I had create a website, selling my photos with my prices, but I had to use some advertising to be known in the internet world.
The t-shirt parameter can be very profitable indeed. I am recently in this kind of business and already I have a descent profit.
What you said about “go out and shoot” it is the best advice you can give. By my side, I can add “less photoshop for better results”!
As always I enjoy reading your posts! Resteemed...