The best thing to do is to use a brake and clutch cleaner spray, give the back of the block a good spray with it to dissapate the old oil and rub as much as you can away.
top up and let the engine tick over for a long while until normal temperature is reached, you can give it some revs too... Inspect it again, it should be more obvious where the leak is coming from then. Wind and air pressure as you drive can force the drops of oil to spray upwards and travel along leading edges. It defeats the natural gravity effect which is why it's difficult to tell otherwise.
Give the sump area a good spray and clean too.
My guess by the pics could be crank oil seal possibly depending on how long the replacement engine has been standing they perish quite easily without the usual heat and cooldown process of regular usage.
The garage really should have checked all this prior to releasing the car to you mate in all honesty.