What would the relative path of the following url be?
http://domain.com/wp-content/themes/themename/images/breadcrumb%5Fsep.png
What would the relative path of the following url be?
http://domain.com/wp-content/themes/themename/images/breadcrumb%5Fsep.png
/wp-content/themes/themename/images/breadcrumb%5Fsep.png
"relative" to the host.... I think that is what you mean, that is the common meaning.
Relative to the root of your domain? /wp-content/themes/themename/images/breadcrumb%5Fsep.png
Relative to the Themes folder? themename/images/breadcrumb%5Fsep.png
It depends on where you are actually placing the link. Pretty much just remove the common parts of the path.
Depends on where it's being called from hence the term relative, it's relative to where it's being called from.
If you're in a sub-folder of your root other than wp-content then the relative path would be
../wp-content/themes/themename/images/breadcrumb%F5sep.png
In the source html file could be relative to some path defined by base href, see:
<head>
<base href="http://domain.com/wp-content/themes/themename/images" />
</head>
then
<a href="breadcrumb%5Fsep.png">text</a>
Anyway, please be more precise with your question to get better answers :)