I won't try to describe it in detail. You can look at the FAQ linked here and will find the details are about 2/3 of the way down the page. In general, any pictures I post are on my computer and nothing is needed except to make sure that they are small enough to post and then, in the message composition process, I select the file in from the message editor and it is done.
For example I am now using the "Upload embedded file: Browse" function at the top of the editing window to embed a wonderful picture of me in high school. A real beauty that!
And it is done...