1. I'm a power blogger, so I try to upload postings and pictures every day.

2. I'm a powerful blogger, so I try to upload postings and pictures every day.
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Q1: Can we use 'powerful' in place of 'power'?

Q2: What does 'postings' mean? Does 'postings' mean 'posted messages' or 'posted documents' or 'posted data'?

2 answers

I guess you can use "power" and "powerful" interchangeably in this context, but "powerful" comes across strangely to me.

Each message added to a blog as a separate entry is considered a post or a posting, yes.
In this case, "power blogger" should almost be treated as a single term. Sometimes in American English, "power" is combined with a noun to indicate the person is very skilled or does the activity a lot. "Power user" is a common term in computer science to refer to a user that has advanced knowledge or needs access to a lot of advanced features.

For instance, most databases or search engines have a basic search function that most people use. Most databases also offer an "advanced" search feature. This advanced search box is more complicated, so many average users lack the skills to use it. Those that are skilled enough are sometimes called "power users."

Powerful does not really capture that meaning.