Texture could make this look so much better.

Also maybe try a render -> lighting effects filter to darken some edges of the image.

For texture try sxc.hu and Textures.

Make a biiiig canvas, 5k x 5k, whack a load of texture layers on ( all diff )

Use a soft edged eraser on 20-30% opacity, and click round the edges to get smoothly blended textures.

Then simply apply them how you like to the image ( or parts, don't just overlay it as one layer, do each bit individually!)

When using textures try to get them all of a similar sharpness / brightness / zoom