How do I upload custom emojis to Trello?

[Goddess]

Several years ago, I was able to upload image files and use them as custom emojis on my boards. Even now, I can still use the ones I uploaded years ago. However, I can not find where to upload more. 

I googled and looked everywhere I could think of for an answer, and all of them said there was an option when you click on the emoji symbol in the text boxes. I looked in multiple places, and while I can navigate to a dropdown menu that has all the basic emojis as well as my old uploads, there is no option to upload more. 

Is this a paid subscription benefit? I used the free trial and didn't find anything different. 

Please let me know, as this tool is so useful to me and provides excellent visual aids for me as I work.