simple just modify existing styling:
comment out sprite specific:
.postbit_buttons a.postbit_reputation_add span {
/* background-position: 0 -20px; */
}
add image specific:
.postbit_buttons a.postbit_reputation_add span {
background: url('images/thumbs-up.png') top left no-repeat;
}
just for example using an externally link image just for an example if say you were via browser edit and wanted to see it display quick before applying actually css file edit*:
.postbit_buttons a.postbit_reputation_add span {
background: url('https://icons.iconarchive.com/icons/google/noto-emoji-people-bodyparts/16/12008-thumbs-up-icon.png') top left no-repeat;
}
* though you would use internal image like below in your actual global.css stylesheet edit:
.postbit_buttons a.postbit_reputation_add span {
background: url('images/thumbs-up.png') top left no-repeat;
}
and then edit further for your specific usages.... also note fyi the most optimal is using 16px by 16px icons....
so just for example if memory serves at famfam being the original icons majority used in mybb back in the days when we were not using sprites and had individual images, umm, yeah...
ie:
http://www.famfamfam.com/
that is just one of many quick ways to find say various 16px by 16px icons to use in buttons or what not...
so in theory just take the image you are hoping to use noted above and make a 16px by 16px variant of such and then modify provided example to reflect your desired image and path...
** ooh, that reminds me that maybe I should re-add my old tutorials in regards to replacing all annoying board sprites with individual images whenever I find the free time to do so again, lol, though if memory serves I think you are correct that back when I wrote the tutorial to replace all, I did not add the postbit buttons to that tutorial as the tutorial was based off classic developmemt and in classic I had opted to keep the buttons sprite as the only sprite remaining in theme... makes sense as in classic days we did not have button icons but had button images and such would make sense that I opted to keep existing buttons sprites as a time saver on that specific project in 1.8 though I will have to dig up the old tutorial to confirm if I did leave that out in such so yeah that reminds me to one find the free time to find & re-add that old tutorial and two then add the one part I left out in that tutorial if I did, haha!
Best of luck! Happy editing!
~ V