if you want parts of said rectangle to be fully transparent, you can easily do that with a .gif image. If you want a semi-transparent .gif image, like the above post suggested, you will need a work around to make it work in the .gif format, since it's not fully supported.
It is however fully supported in other formats, such as .png, but this format will create images that are too big to be used as signatures.
This post was edited by Merscil on Jul 19 2015 01:25pm