When should I use "了" in “把”字句 and where should it be placed?
Compare these sentences:


  
  我把课本放在桌子上。 
  他把病人送到医院。
  我把课本放在桌子上了。
  他把病人送到了医院。
  


Sentence 1 and 2 don't indicate the tense and tell whether those events happen in the present, past or future. Sentence 3 and 4 clearly indicate those events happened in the past because they have 了. 

Practically, you could put 了 after verb. If you see a prepositional phrase in the end, you can attach 了 at the end of the prepositional phrase or sentence.  

For example, 


  
  我把课本放在桌子上了。 在桌子上 is a prep phrase, so you attach 了 at the end. The same thing for 姐姐把照片存在电脑里了。 我把我的新书丢在公共汽车上了.
  他把病人送到了医院。in this case, it doesn't have any prep phrase in the end, so you can put 了 after the verb 送到. In 小明把礼物送给了妈妈。送给 is the verb, so
  you can put 了 after it.