I see... Well, this is quite a complex question, since it envolves a lot of steps and possible solutions. So I will try to give you a few suggestions...
Regarding the detection of interesting elements, openCV is a possibility. But I believe it's more focused on clear features like smiles, faces, contours, et cetera.
Since you are starting already with abstract paintings, you could define different criteria. Just like when you when with luminosity, maybe tint, kernel area, contrast, and so on. Another ideia would be, prior to playing around in OF, creating masks for your images (in Photoshop or similiar), and then use those masks to play around with your image.
About the mesh and texture manipulation, and since you want to play with both in realtime, if your window size starts to get a bit larger, you'll probably go out gas pretty soon. With you go only with CPU, of course.
So I would advise you to go GPU and check this tutorial. Especially the Textures as Data (e.g. Displacement). But instead of using a noise texture, you could create an ofFloatImage, based on sound input.