now, rename your current module folder
and then git clone your fork, and do npm install if the original asked for it

now, copy the changed files from the renamed folder to the new one.

if all is good
then we can work on getting the changes pushed up to your fork

fork original
download your forked copy
make changes
git add/commit/push your chnages back to your fork…

File: modules/MMM-Volumio/MMM-Volumio.js (the bit I changed at the end)

        var data = this.volumioData;
        var amendUrl = "";
        var item = document.createElement('div');
        var image = document.createElement('img');
        var artists = document.createElement('div');

        if (data['service'] == "mpd") {
            amendUrl = this.config.volumioUrl + data['albumart'];} // if local playback put volumio url +  albumart url 
        else{
            amendUrl = data['albumart'];}                   // else if web radio just albumart url

        artists.className = 'text';
        artists.innerHTML = '<div>' + '<br />' + data['artist'] + '<br />';
        wrapper.appendChild(artists);

        item.className = 'smalltext' //'mmm-volumio-item';
        item.innerHTML = '<div>' + data['album'] + '<div>' + '</div>' + data['title'] + '</div>';//I put 'albumart' here to prove I had the url
        wrapper.appendChild(item);                          // display artist, album and title info

        image.src = amendUrl;                               // set url to the image
        image.className = 'image';
        wrapper.appendChild(image);                         // display album artwork
        return wrapper;
    }
});

File: modules/MMM-Volumio/MMM-Volumio.css

.region.fullscreen.below {
    position: absolute;
    height:   100%
    width:    100%
}

.mmm-volumio-item {
    text-align:  center;
    line-height: 40pt;
    font-size:   95%;
    color:       LightGrey;
    position: absolute;
    height:   100%;
    width:    100%;
    border-right:  24px;
}
.image 
{
    position:    Fixed;
    bottom:      0px;
    left:        0px;
    width:   100%;
}

.text
{   
    text-align:  center;
    line-height: 60pt;
    font-size:   160%;
    color:       DimGrey;
}

.smalltext
{
    text-align:  center;
    line-height: 40pt;
    font-size:   100%;
    color:       Grey;
}

As you can see it needs to be shifted about a little to look good.

        var data = this.volumioData;
        var item = document.createElement('div');
        var amendUrl = "";
        var image = document.createElement('img');
        if (data['service'] == "mpd") {
            amendUrl = this.config.volumioUrl + data['albumart'];} // if local playback use volumio url +  albumart url 
        else{
            amendUrl = data['albumart'];}                   // else if web radio just albumart url
        image.src = amendUrl;                               // set url to the image
        image.height = 600;
        image.className = 'image';
        item.className = 'mmm-volumio-item';
        item.innerHTML = '<div>' + data['artist'] + ' • ' + data['album'] +
                         '</div>' + '<div>' + data['album'] + '</div>';//I put 'albumart' here to prove I had the url
        wrapper.appendChild(item);                          // display artist, album and title info
        wrapper.appendChild(image);                         // display album artwork
        return wrapper;
    }
});

Here is my messy code, I don’t really do any coding so I was winging it.

on my phone

If albumart value starts with http, then no further operation is needed and the resulting url will show an albumart
Otherwise, prepend the string formed by: http:// + IP ADDRESS of Volumio device. Example: http://192.168.1.22/albumart?