After attaching an image, the "name" of the image appears as a link, and you can copy that link as your "URL", or click it to open the attachment in a new tab and copy the URL from the browser's address bar.
For this forum, it will look something like "https://www.chinacarforums.com/forum/attachment.php?attachmentid=xxxxx&stc=x&d=xxxxxxx"
I have attached a picture below named "Sample_1.jpg". The URL in this case is:
Paste that within [IMG] tags and you're done! Note that the thumbnail of the attachment will be visible too down below.
P.S.: Note that you can attach
up to five images with a post, but this forum only allows four inline
images in a post, so one will have to be left out (but still visible as thumbnail).