Confirmed:
Images added by Plus! seem to be mirrored when using Messenger in a RTL language. I have tested it with Messenger 2011 Arabic, judging by the OP's screenshot he is using Messenger 2009. I initially tested this with Plus! 5.0, but the same issue after downgrading to Plus! 4.9. Which makes me wonder why this issue only seems to be reported for the first time now... if WLM 2009/2011 and Plus! 4.9/5.0 were all affected by it, shouldn't we have seen at least a few reports over the years?
When using a skin the images from the skin are mirrored. However, even without a skin Plus!' own icons are mirrored to, which leads to problems like in
reversal the Messenger plus "5" Icon ..!.
(Messenger 2011 Arabic + Plus! 5.0 without a skin)