How to change WebMail logo?

You should modify styles.css file of the appropriate skin. Some skins shipped with the product have logo, other ones don't. Let's take Hotmail_Style for example as it has the logo.

The following class defines the logo height:

.wm_logo
{
    height: 58px;
    border: 0;
    overflow: auto;
}

The following one - logo image source:

.wm_content
{
    background: url(header.jpg) no-repeat center top;
}

Actually, the height of the default logo image is greater than 58px, but it's limited to 58px on all screens, except login screen. So the image source is defined in one wm_content class, but size of the logo container which limits the logo height in wm_logo class.

So, if you need to change the logo to my_own_logo.jpg with height 64px, the classes should look like:

.wm_logo
{
    height: 64px;
    border: 0;
    overflow: auto;
}

.wm_content
{
    background: url(my_own_logo.jpg) no-repeat center top;
}

Additionally, the following class creates a gradient which smoothly blends the logo with the background (you may need to change it too):

body
{
    margin: 0;
    padding: 0;
    width: 100%;
    background: #336699 url(gradient.jpg) repeat-x top;
}