@
Dragonchompers "URL" means the page's address. The long box at the top of your browser that has all the letters and numbers, and they change whenever you click to go to a new page? That's what a URL is. You can copy and paste it, and by using the code in my first post, you can link someone else to the same page.
If you've already found the image you want online (provided you have permission to use it, of course) you just have to open the image as its own tab and copy the resulting URL on top, OR right-click the picture and select "copy image address." Paste it in between the boxes labeled "img," and make sure the URL ends in the proper file extension (such as .png, .jpg, or .gif) or else it won't work.
If you have a picture in your files that you want to post online, you need to upload it somewhere else first. If you have a Tumblr account, you could save a draft of your images so they have a code, but cannot be seen by other users. If not, I have to recommend the easy-to-use site
Imgur, where you can easily upload lots of private images so that you can post them with image codes like that.
Here's an example, featuring a picture I posted on Twitter of my cat questioning a gnome.
Code:
[img]https://pbs.twimg.com/media/C6l21i1VwAAKqa-.jpg[/img]
EDIT: If you are reading this thread, please
do not respond or ping me for further help on posting image codes. Make a new thread with the "+New..." button at the top right of the page instead, and ask your new question with as many details as you can provide (posting the broken code you're trying to use is a big help). You're also likely to get answered by someone faster and more knowledgeable than me.