Skinny and thin are pretty much the same and these two words can be used interchangeably. An example is "You are very thin" and "You are very skinny". There are only slight differences between skinny and thin and as an native English speaker I can only think of a few. I would say that skinny is a word that a younger person would use whereas thin is a word that an older person would use. For example a mother would say to her daughter "You are getting too thin!" whereas a younger person might say "Girl, you are so skinny!". The first is not meant as a compliment whereas the second is. But that is not to say that thin is not a compliment whereas skinny is, it is because of context that one was a complement and the other was not. For your purposes it is ok to use them interchangeable it will not create a problem.

Strictly speaking, I would agree with the definitions provided by yuyan不通. Thin is used to describe 2D objects or a profile; skinny is used to describe 3D objects or forms. BUT, I think this misses the context of the question. In the vernacular, thin and skinny are both commonly used to describe a person's body. I think this is the context of Yi-Ying's question. In this case, I would argue that the terms 'thin' and 'skinny', to describe someone's body shape, are very nearly the same. 'Thin' to me has a slightly more positive connotation; 'Skinny' is used to describe younger people (children) or if used to describe adults, has implies a slightly more negative meaning. Perhaps the most positive way to describe a person's body is the term 'slender'.

I'd say that skinny is negative and refers to something called anorexia. Anyway, jon's pretty much spot on so just read his answer. :P

skinny"瘦" is the opposite of fat; and thin"薄" is the opposite of thick. they are NOT interchangeable! now many people are beginning to misuse skinny in the english lexicon (like many other words) and it's gaining more popularity in use of describing all kinds of inanimate objects. basically, thin is used to describe shape (2-dimensional), whereas skinny is used to desribe form (3-D). the two most common examples for use of skinny i can think of are: she's really skinny and probably only weighs 50kgs soaking wet just tell me the skinny of that report... so if u try to use it any other way, it's probably going to be wrong hahaha :P let's see and for thin: the butcher cut that slice of ham so thin, i can barely taste the meat the material was so thin, it almost came apart when i held it in my hands