You can't have transparent backgrounds, which is what you need for a bitmap
that has circular details. Visio ONLY handles rectangular bitmaps.
If you are talking about extra details beyond the circle, well, you need to
crop the bitmap. This is easily done in Paint or any other bitmap editor,
and Visio even has a crop tool on the Picture toolbar. This will let you get
the rectangle to tightly enclose the circle, so at least you'll have less
junk outside of your circles.
The crop tool lets you hide stuff out on the edges of your bitmap.
Hope we're getting closer...
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Hope this helps,
Chris Roth
Visio MVP