The reply boxes in all the message boards are intended for quick textual replies. It would have been nice to give them the same funtionality as the more ornate post editor, but this would have made the page too heavy, complex and difficult to understand.
To insert a graphic from another website such as glitter-graphics.com, you can simply copy and paste the HTML or BB (forum) codes provided.
To insert a graphic either from this or another website, if you only know the direct link (sometimes called URL or internet address), you need to add your own HMTL code. This code is known as the IMG tag. You can google any HTML tutorial and look it up. The IMG tag has the following format:
<img src="your URL here">
so your HTML code might look something like this:
<img src="https://your graphic.gif">