Currently, ShinobiCharts only supports a solid colour as a background. If you want vertical stripes on your chart, I would look at adding annotations to your chart. Specfically, the method +verticalBandAtPosition:andMaxX:withXAxis:andYAxis:withColor: is what you are looking for!
To have it appear as though an image is the charts background, you could set the charts background to be [UIColor clearColor] and then place an image behind the chart. You can get the frame of the chart or plot area (to use as your UIImageView's frame) like so:
// Chart frame.
CGRect chartFrame = myChart.frame;
// Plot area frame.
CGRect plotAreaFrame = myChart.canvas.glView.frame;