Display photos horizontally in feed

This can be achieved with some custom CSS. You can either add the custom CSS to your theme or use a plugin. Jetpack has a custom CSS module if you use that.

The CSS would be something like:

.instagram-pics li {
    float: left;
}
.instagram-pics li:before {
    content: '';
    padding: 0;
}

p.s. There is no version of this plugin or WordPress at 4.9.1.2. This leads me to believe you may have been charged for code that is free.