Hi @mr_fme 

You could try changing the Data Values parameters, particularly the Overlapping Values parameter. By default the last value in will win, so if a raster with empty space comes in after a raster that has meaningful values in that area, the empty space will overwrite the meaningful values since they arrived later. Changing the data value parameters may help with this, here are a few things you could try:

• If your data has an alpha band, setting Overlapping Values to Composite Using Alpha Band will likely have the best result.
• If your data has nodata values, ensure that Nodata Overwrites Data is No. You may also need to set the no data value before the RasterMosaicker. 
• If your data has neither alpha nor nodata, you could try setting Overlapping Values to Minimum, since white would be (255, 255, 255), so any real value would be less than that.

Please let me know how this goes and if you have any questions.

Crystal 

Thank´s