You can use a url for an image if you know the html to associate with an image AND the image is accessible. Most likely you would be using a file/photo sharing site like Instagram, flickr, youtube, shutterfly, my personal favorite: smugmug. Technically you could use your Google drive. You need to bracket the image with BB Code [url]and your http addres and[/url].
I believe the preferred mechanism is to use an image that has been uploaded to the site's gallery area.
You can get to the gallery several different ways:
(1) At the top, opposite the "board index" on the right side is the word gallery. Click on gallery and you go to list of images.
View recent images • View random images • View recent comments • View top rated images • View your images
(2) Above in the editor, the last button above the edit box is "gallery". It takes you to gallery as a separate popup. It has access to your gallery, recent images and upload(ed) images.
In any case, you need to pick an image. You really can pick anyone's image if it is allowed.
When you select an image, a bigger display with the image and other information is displayed.
Below each image is a way to post that image on the site:

[click to make this bigger]
Below the selected image are several ways to display the image in your message eg the imageBBCode. Copy one of the texts and paste it into your message. For instance,
[album] <the number from BB Code> [/album]
will show the picture. While you are editing, the above text show up ( when it is a number with no spaces ) When in preview or others are reading your post, it is the image.
uploading:So, lets load an image.
First log in as you.
Second go to your profile. ( Click on board index: It should display "User Control Panel" right below it. Click on "User Control Panel" )
Third: Go to the Gallery tab : Last tab from left edge or first tab on right edge
Fourth: Go to "manage personal album" Second vertical tab
Fifth: Select an album or subalbum or create a new subalbum Use the create subalbum to create a new album.
Sixth: Pick a specific subalbum by clicking on it.
Seventh: upload an image using the "upload image" button. You have limits on size and type of files.
Eighth: add any comments about the images, etc AND save the image.
You now have an image to paste into some posting.