Try using stock images at different layer modes, my favorite is soft light, then adjust the opacity of each accordingly. Doing so will help fill the negative space(empty space). You should try using fractal renders around your focal point to help with blending, just erase parts of the fractal renders as needed until you have the desired effect. Try this with the layer mode on normal first, but if you are unhappy you can experiment with different layer modes after. Blending is one of the most important aspects of gfx design.
Always Remember, "Good design is obvious. Great design is transparent." - Unknown