Hello i think it should nice to use a custom css for forum's icons or catégory's icons, because in one big forum, the admin may want use a single image with a css position..
Example:
I have 56 forums and 40 category so it's equal to 96 icons !!! Time to load, woow, this increases the server load and the server bandwidth and the client caching size...
While if i use a single image who contains all my icons, i can use it in all forum with a custom css position...
With a nice css:
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span.forum-image

{ background: url("[root]/images/my_custom_forum_icon.png"); background-position:16px 0px; background-size: 800px 16px; /*It can contain 800 icons in one line and it is very light !!!*/ height: 16px; width: 16px; position:absolute; }

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Maybe some admins don't understand what is a css Class but other can use it with pure ease