Assuming your statistical map is lower resolution than the background image, I would strongly encourage you to threshold the data AFTER it has been upsampled, rather than BEFORE. Due to spatial smoothing, the non-significant neighbors of significant voxels are very likely to be ALMOST significant. However, this information is lost when you threshold the data. If you plan to upsample data that has been already thresholded, I would male sure you have "Load Smooth Overlays" unchecked in the "Options" pulldown of the "Layers" panel. The overlays may appear jagged if you do not smooth them, but smoothing data that has been masked can be misleading. You want the interpolation to work with all the available data.

I mention this briefly in the cluster threshold scripting page.