[blind-eddie]

Unfortunately all custom bbcode uses the phrase Wrap [{1}] tags around selected text
Editing that will change all "atl" for custom bbcode to what ever you change it to and I can not find where to change it in the plugin for the table bbcode itself.

You could remove the image url from the table bbcode and manually edit the editor_toolbar_on template and add the code to show the table.gif, adding the "alt" text you want.