I wil appropriately use images to step you through it!
In the toolbar above you message box, click on the icon circled in red.
View attachment 41140
There are two tabs at the top of the box that opens. Mine defaults to URL, so you would click on the "From Computer" tab if you are uploading a picture from your computer.
View attachment 41141
On the next box that opens, click on "Choose File" and then locate and select the image you want to insert.
View attachment 41142
The final step is to "Upload Files".
View attachment 41143
After you upload, you'll see a message similar to ATTACH=CONFIG]41143[/ATTACH in your message. That is where the image will display in your post.