You can eager-load elements that were already loaded (like an Entry or Product that Craft set up for you "outside" a template) like this:

{% do craft.app.elements.eagerLoadElements(
  className(product),
  [product],
  [
    ['variants.variantImage', { withTransforms: ['thumbnail'] }],
  ]
) %}

This will load + set any downstream elements, just as though you'd used it on the original query.

A couple notes:

• className() is used to capture the fully-qualified name of the Element class. It's required to let Craft know what kind of eager-loadable properties the specific Element Type has;
• [product] is an array, because eager-loading is always done in "bulk," even for a single source element;

You can do a similar thing directly on the Variants, if you've got a reference to them, already:

{% set variants = product.variants %}
{% do craft.app.elements.eagerLoadElements(
  className(variants | first),
  variants,
  [
    ['variantImage', { withTransforms: ['thumbnail'] }],
  ]
) %}

You can eager-load elements that were already loaded (like an Entry or Product that Craft set up for you "outside" a template) like this:

{% do craft.app.elements.eagerLoadElements(
  className(product),
  [product],
  [
    ['variants.variantImage', { withTransforms: ['thumbnail'] }],
  ]
) %}

This will load + set any downstream elements, just as though you'd chained an equivalent .with() on the original query.

A couple notes:

• className() is used to capture the fully-qualified name of the Element class. It's required to let Craft know what kind of eager-loadable properties the specific Element Type has;
• [product] is an array, because eager-loading is always done in "bulk," even for a single source element;

You can do a similar thing directly on the Variants, if you've got a reference to them, already:

{% set variants = product.variants %}
{% do craft.app.elements.eagerLoadElements(
  className(variants | first),
  variants,
  [
    ['variantImage', { withTransforms: ['thumbnail'] }],
  ]
) %}