The first problem is "guessing".
Let's stop with the guessing and let's understand what "relative" actually means.
When you have something "relative" there is always a comparison between "something" and something else, in this case is the comparison between two paths.
When you are referencing a path "relatively to another, you have to imagine to start from one path and write the route you have to take to go to the another one.
Let's use an example:
c:\users\emanuele\documents\elkarte\themes
if I want to go from the path above to let's say:
c:\users\emanuele\webserver\myforum\themes
I have to:
1) go up one level (to elkarte),
2) go up another level (to documents),
3) go up another level (to emanuele),
4) enter "webserver",
5) enter "myforum".
Now, when you try to speak the "pathish" language (in the MS Windows dialect), "go up" is translated to "..\" (two dots and a backslash).
So, from the first path to the second, using a relative path you have to write:
..\..\..\webserver\myforum
Does it sound about right up to here?
Okay, now let's move to the web.
The web speask another "pathish" dialect, the "unix" one, in which case, the backslash (\) is replaced by a slash (/). Easy, right?
Good, then let's assume the URL to the css file is:
http://myforum.de/elk/themes/mytheme/css/_light/custom_light.css
then we have our image at:
http://myforum.de/elk/themes/mytheme/images/_light/example.png
The principle is the same, starting from http://myforum.de/elk/themes/mytheme/css/_light/ :
1) go up one level (to css),
2) go up one level (to mytheme),
3) enter into "images",
4) enter into "_light".
This translates to:
../../images/_light/
So, in your custom_light.css file, to pick images in the images directory you have to write:
background-image: url(../../images/_light/example.png);
The same thing you already wrote, but now you know why you wrote it that way.
Now, let's see analyse the "block" situation.
When you visualize a block, what is the URL in your browser? Most likely something that looks like:
http://myforum.de/elk/index.php?some=garbage
right?
Okay, the relevant part here is anything up to "index.php", so:
http://myforum.de/elk/
Now, this is the path you start from, from here you have to go to your image that resides at:
http://myforum.de/elk/themes/mytheme/images/_light/example.png
So, how should it look like the relative path?
hmm... do you have to go up of any level?
I'd say no, what you have to do is follow the directories:
1) enter into "themes",
2) enter into "mytheme",
3) enter into "images",
4) enter into "_light".
Translating to "pathish" results:
themes/mytheme/images/_light
So, your image tag should be:
<img src="themes/mytheme/images/_light/example.png" width="98%" height="auto" alt="Example">