I think you may have mis-understood the problem: there isn't a single background-image property in any Prosilver css files or inline styles that puts a background-image to any DIV under ".buttons" class (or at least it seems so). In other words, if you remove these definitions, nothing happens! Just re-checked and that is the case.

All button related background images are targeted to SPAN tags. For example, colours.css:664:
.reply-icon span {
background-image: ...
}

So, buttons.css:17 is a redundant line
.buttons div {
float: left;
margin: 0 5px 0 0;
background-position: 0 100%; /* redundant */
}
because in markup this definition affects only to "DIV.buttons DIV.reply-icon" and not "DIV.buttons DIV.reply-icon A SPAN" where the background image is.

Also buttons.css:25 is a redundant line
.buttons div a {
display: block;
width: 100%;
height: 100%;
background-position: 0 0; /* redundant */
position: relative;
overflow: hidden;
}

But as I said, the problem is not a big one, it just confuses me that there may be something in this button hover image technique that I don't understand.

Ah, I see what you mean. Its not redundant, its just unused.

.buttons div a is not unused though. It is the same as default value in browsers, but to be on safe side it is better to set it. Users often change default values for stuff in stylesheets for their websites when they add custom header/footer, so it is not a good idea to rely on browser default value.

It has already been fixed in phpBB 3.1

Okay, thanks for the explanation

Thanks for reporting. If you'll find any other similar (or not similar) bugs, please report them as well

I think this ticket should be closed because bug has been fixed (unintentionally when rewriting buttons) in 3.1
Fixing it for 3.0 might break existing mods and third party styles.