Since we removed support for supplying CSS style attributes to the image tag in wiki markup, we have replaced one of the CSS attributes we used to use in our own docs with a specific attribute. This is called bordercolor. It is used in conjunction with border (otherwise there would be nothing to colour). An example is -

!baboon.png|border=3,bordercolor=red!

I think the 2.6 and 2.7 documentation will need to be updated.
e.g. http://confluence.atlassian.com/display/DOC/Displaying+an+Image