category archive pages with a description, an image and the relevant posts.
You'll be just fine. Having the description in the archive pages and changing text below it as you generate new posts is perfectly acceptable behavior and you should not see any penalties or drawbacks UNLESS you change the category names and/or permalinks schema in the future.
Remember, a URL is a URL is a URL as far as Google is concerned. There is no functional difference between
www.domain.com/your-page/. So internal and external linking strategies are unaffected by your choice and only the content matters.
Pages in WordPress are nice because they always take
www.domain.com/page-slug/ format and unless you change a slug manually, it never changes.
Categories, Tags, and Posts are meant to be a little more mutable and people do tend to muck with them as well as the overall permalink structure over time. A good example of this are the custom structures for category and tag bases (see
Settings | Permalinks). Once you start to be indexed by Google, all of those items become set in stone for you as changing them would then involve setting up 301s and living with whatever temporary ranking hit you take from Google until it recalculates and re-indexes everything.
So as long as you are happy with your category names and slugs, you should be fine.