'you guys' means, you people (i.e. 'you' is plural). If we omit the word 'guys', like this,Originally Posted by wendy
1. pictures of you partying
the resulting meaning is singular 'you', like this,
2. pictures of you, John, partying.
The pictures are of one person only: John.
The word 'guys' is important because it tells us 'you' is plural, and not singular.
Next, if we change 'you' to 'your', like this,
3. pictures of your guys partying
the resulting meaning is the 'guys' belong to you. That is, your friends, they alone, were in the pictures; you weren't in the pictures.
If we change 'your' back to 'you', like this,
4. pictures of you guys partying
'partying' functions as a modifier. It describes what 'you guys' were doing in the pictures.
Consider the structure of the phrase:
5. pictures of you guys partying
Structure: pictures (that were taken) of people (doing something)
Participles generally answer the question "Doing what?", whereas gerunds answer the question "What?"
EX: Partying is fun.
Question: What is fun?
Answer: Partying. (Gerund)
EX: A partying animal
Question: An animal doing what?
Answer: Partying. (Participle)
EX: Pictures of you guys (Doing what?) partying. (Participle)