Here is what I've found...
The BBCode plugin was not necessary to include a PayPal button in the description area of a photo. Just plain bbcode for URL and IMG will do the trick. Below are some links I found helpful.
First, when you create a PayPal button the display defaults to 'website' code. Clicking the 'email' tab shows the URL needed for a link to your now saved button. The link will look something like this.
Next you will need a link to the appropriate button image. Buttons for English language websites are at: https://developer.paypal.com/docs/classic/api/buttons/
Other language button files can be found at: https://developer.paypal.com/docs/classic/archive/buttons/
Put the button link and button image together like this..
Paste this ^ into the File Description, apply the changes, the functioning PayPal button should now be in your Description box
I found most of what I needed on this PayPal page. https://www.paypal.com/webapps/mpp/get-started/add-payment-link-to-facebook