In brief pictures for articles (Posts or Pages) are stored in one of three places for the website and can also be external to the website (ie on another website).

If the picture is going to be in the District Photo Gallery, [for large photos and big sets not usually used on a page, ie. from the 1000 photos of the District Conference] we use the address of the picture we want in the page from Flickr where we store them.

For regular articles we use the Media library or
the Rok Gallery rokgallery.desc (for those backgrounds and special effects plus search options)

Log In to the website using your password.

Navigate to your article, Latest Posts for example and click on the title.
At the bottom of the article you should see an Edit this entry. link, click that.
There are many ways to get to your article choose the method you like best

Now in the Editor you can adjust your article.

Lets assume your picture is in the same place as the one you loaded.

Click the image you want to change to highlight it.
Click Add Media
Select Edit Media
Select Edit Media

Click Media Library (assuming the picture you want is here)
Select the photo to replace the one you have.
You may want to choose a good size, say 200 x 300
Click Insert into Post

Media_Lib

Now you should be taken back to the editor.
Update your work

Check the website to see if it is OK.

If you have a new photo to Add use the  Add Media
Upload Files and the drag and drop any images on your computer onto the screen.
Once the file is uploaded it is available for use as previously described.

Here is a video which I hope helps.  Help Video