axios.get inside a for

The easiest way to solve this is using async/await , If you have not heard or used this you can read this page .

Taking your sample code you need to declare getImgProducts as async :

   async function getImgProducts(line_items, cd) { ... }
// ^^^^^

Once that is done, you can use your for as if it were any other synchronous function.

async function getImgProducts(line_items,cd){

    let responses = [];  // Almacenamos todas las respuestas en un array

    try {
        for(let i = 0; i < line_items.length; i++) {
            let url = '${Shopify.appurl}products/${line_items[a].product_id}.json?published_status=any&fields=id,images';
            responses.push(await axios.get(url));
            //             ^^^^^ importante añadir esta parte
        }
    } catch (err) {
        // En caso de algun error
    }

    return responses; // Retorna el array con las respuestas, cambialo como necesites

}

Just remember that when a function is asynchronous, a Promise is always returned.

Ex:

function callerFunction() {
    getImgProducts().then(function(responses) {
        //           ^^^^ Utilizamos then para 
        // acceder a los datos una vez que ha sido resuelta.
    }); 
}

Finally it's like this:

async function getImgProducts(line_items){
    let responses = [];
    try {
        for(let i = 0; i < line_items.length; i++) {
            let url = '${Shopify.appurl}products/${line_items[i].product_id}.json';
            responses.push(await axios.get(url));               
        }
    } 
    catch (err) {
    } 
    console.log('***************************responses', responses[0].data.product.image.src) ;
    return responses;  

}

And the one called:

getImgProducts(line_items).then(function(imgsProducts) {....}