There are the following ways to convert the geotiff orthomosaic that PIX4Dmapper generates into .jpg.
Using an image editor/viewer:
1. Convert (Save as...) the geotiff into .jpg using an image converter such as IrfranView. http://www.irfanview.com/.
2. Change the extension of the tfw file that Pix4D software generates to jgw.
3. Place the jpg, the jgw and the prj folder in one folder. This folder now contains the georeferenced jpg.
Using Global Mapper (GIS commercial software):
1. Open the geotiff file in Global Mapper.
2. File > Export > Export Raster/Image Format.
3. Select JPG. Make sure that the options World File and PRJ are selected.
Using QGIS (open-source GIS software):
The following instructions could be useful: http://gis.stackexchange.com/questions.