It is not at all true that all pics gave to be online to post. Just in some cases, such as PMs.
Posting them in a blog is the same as posting them in a regular thread.
The bar above your post that contains font options, if you look at the 3rd from farthest right you'll see a square icon with a black border. Click that then follow the instructions from there. It gives the option of online pics or uploading pics from your pc.
Also, in the future, if you have any sort of tech questions we have a forum set aside for this. The User2User Tech forums is for posting any kind of such questions, even ones not about the site.