Is it illegal to post nude pictures online

First, this question is improperly categorized as a personal injury matter -- it clearly belongs in the criminal defense matter.  The short answer to your question is yes, you can be arrested for the conduct that you have described.  The particular charge will depend on more specific facts, for example, whether your girlfriend was over the age of 18, whether you posted the pictures as a means to threaten, harass, extort, or blackmail her, etc.  In any event, I suggest that you consult with a local attorney to discuss this matter in greater detail and to minimize any criminal liability that you may potentially be exposed to.