If you want to post directly from Smugmug (ie, the picture says on Smugmug and is served from there), you need to get a link from Smugmug via the "share" icon (square with up pointing arrow; see screen shot below) .. select embedded link, pick your size, and then cut and paste it into the message text with
image start end tags around it ... see screen shot of my VAF text source, also below.
If you want to simply post a photo that's on your machine (like I did here, below) use the "attach files" function at the below the text box when you create a new message. These photos only show up as small thumbnails, and are copied to the VAF site after posting.
OR, you can choose the "BBCODE" option in Smugmug and then you don't have to do any "codes" on VAF. Just copy paste directly from the BBCODE text into VAF.