Render image while keeping ratio in Echarts label

I have a bar chart that displays operators statistics, and I want to use the logo of each operator as a label. Currently, I have managed to make it display the image, but only with a fixed ratio.

Examples:

With no height and no width

With width: 110, height: 20

With only height defined (20)

 var operatorsChart = echarts.init(document.getElementById('chart-operators'));
    operatorsChart.setOption({
        title: { text: "{{operator}}" },
        tooltip: {
            trigger: 'axis',
            axisPointer: { type: 'shadow' },
            formatter: function(params) {
                return commonTooltipFormatter(params);
            }
        },
        yAxis: {
            ...commonXAxisOptions,
            type: 'category',
            inverse: true,
            axisLabel: {
                rotate: 0,
                formatter: function(value) {
                    const normalizedKey = normalizeKey(value);
                    if (operatorImages[value]) {
                        return `{${normalizedKey}|}`;
                    } else {
                        return value;
                    }
                },
                rich: Object.assign({}, ...Object.keys(operatorImages).map(operator => {
                    const normalizedKey =  normalizeKey(operator);
                    return {
                        [normalizedKey]: {
                            width: 110,
                            height: 20,
                            backgroundColor: {
                                image: encodeURI(staticLink + operatorImages[operator])
                            }
                        }
                    };
                }))
            },
            data: dataToUse.operators[viewMode].map(item => item.operator)
        },
        xAxis: { type: 'value' },
        grid: hGridConfig,
        series: createSeries(viewMode === 'km' ? convertToKilometers(dataToUse.operators[viewMode]) : dataToUse.operators[viewMode], getColor(tripType))
    });

I can fix the height and width, but with my images having different ratios, the rendering is bad.
I can also remove the heigth and width, but there the sizes are wildly different.