I'm not able to add or edit custom BBCodes, so unfortunately I'm unable to do this for you. Though our lead admin gv1.3 might be able to create a custom BBCode using the below HTML code to grab and display the Instagram image.
BBCode Usage:
[insta]{URL}[/insta]
HTML Replacement:
<img src="{URL}media/?size=l">
I'll ask if he can add this to the list of things to do.