In addition to qiv mentioned in the previous post, I found another image viewer which could do some amazing things when viewing transparent images. It is CThruView written in Java.
However, my trial got some problems. The transparent background was replaced by a gray one, and its visibility can only be reduced along with the foreground image by setting the opacity of overall image. The following screenshot shows my test.
At the first trial, I though the fails might be due to the OpenJDK which is the default installation in Ubuntu. So I found the method to install Sun Java 6 but got the same result.
Another failed test was the option ``Click-Thru'' which had no function. Except for these, other options worked properly.