Icons

Icons8

Simplemente poniendo url

https://img.icons8.com/color/48/000000/ubuntu--v1.png

https://icons8.com

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FontAwesome

https://fontawesome.com/

Dentro de <i>

<i class="fas fa-exclamation-circle text-danger fa-3x"></i>

<i class="fas fa-exclamation text-danger fa-3x"></i>

<i class="fas fa-exclamation-triangle text-danger fa-3x"></i>

<i class="fas fa-info-circle text-danger fa-3x"></i>

<i class="fas fa-check-circle text-danger fa-3x"></i>