透明GIFがうまくゆきません...
This is usually due to a browser bug. Make them non-transparent or
accept that a particular browser will not display them very well.
In particular, some browsers do not display transparent GIFs well
in 16-color modes under Windows. You may also wish to respectfully
suggest to users of 16-color Windows that they try switching to
a 256-color mode. Most graphics cards support 256 colors,
but a surprising number of machines are configured for 16 colors
instead.
World Wide Web FAQ