Highslide - unwanted background appears when mousing over around outside of expanded image

Highslide - unwanted background appears when mousing over around outside
of expanded image

I'm having a problem with the expanded image in Highslide. For some
reason, I'm getting some white/gray (#EBF6FF to be exact) background
appearing when I bring the mouse close to the image around the outside.
Here's a link to a screenshot.
It also does it on the sides (a white box/background appears, jutting out
13 pixels on either side, beginning at the bottom edge of the area shown
in the image above), and the bottom, although for the bottom you need to
mouseover outside the caption area, along the lower right and left corners
of the image. The effected area is 13 pixels beyond the top and sides of
the image. I've gone through my CSS and can't find any 13 pixel padding,
and also can't find any references to #EBF6FF. Has anybody seen this
before?
I've used Highslide in the past with the glossy dark border, but I wanted
to switch to a borderless look. I'm not re-using any of the previous HS
code (with the glossy border). I started this one from scratch using the
latest version (4.1.13). Everything else is working fine, the only problem
is with this unwanted background appearing on mouseover. Unfortunately,
I'm still developing this on my localhost, so don't have it live to view
on the web. But I can provide any code that you may need.
Thanks in advance for any help/suggestions anybody can provide!