LOOP offers 3 types of icon:
Icons inherit their size from the current font-size context they're in. Icons are 1em high, widths vary depending on the artwork.
Uses classes from the FontAwesome icon library.
Has inline styles to set the mask-image and the width (in ems, height being 1em)
Mask images use the alpha (opacity) settings within the image. SVGs are best as they will scale to any size.
Picture icons use the same sizing rules as above, but they are effectively just images.
Images can be any format - though raster images (GIF, PNG, JPG, WEBP) will require srcsets. SVGs will scale natively.
There can be an optional 'hover' image