I had a similar situation (
http://pivlab-time-resolved-digital-particle-image-velocimetry-tool-f.33690.n7.nabble.com/Particle-Density-and-Color-td13.html). But in my experience I had similar or even better results with dark particles on a bright background. But I guess this might depend on your setup.
If you, however, still want to invert your images, I would recommend the program IrfanView. There, open your image and press B, add all images to the queue, set the special option "create negative" (or similar). Choose your output destination and hit "start". Done :)