Check your oil (via the dip stick) and look for any milky or bubbly film. If you have that, then you're in for it because the head gasket(s) is done.
As stated above, check the hose at the reservoir to ensure it's not leaking or cracked.
Do a pressure test of the radiator and see if the system holds pressure over night. If not, locate the leak.
Can't tell from the picture very well, but if it's leaking from the front of the block and back, it could also be the water pump. Inspect the gasket around the pump. Also, check the weep hole on the pump to see if it's clogged or leaking.
2004 Ford Ranger 4x4 XLT.....4.0 V6....4.10 rear
2" torsion key crank
235/85/16 Goodyear Duratracs
Gibson exhaust (POR20 coated)