The ImageUrlBuilder class exposes methods for applying image transformations on the Asset URL.
string assetUrl = articleItem.GetAssets("teaser_image").First().Url;
ImageUrlBuilder builder = new ImageUrlBuilder(assetUrl);
string transformedAssetUrl = builder.WithFocalPointCrop(560, 515, 2)
.WithDPR(3)
.WithAutomaticFormat(ImageFormat.Png)
.WithCompression(ImageCompression.Lossy)
.WithQuality(85)
.Url;
For list of supported transformations and more information visit the Kontent.ai delivery API reference at https://docs.kontent.ai/reference/image-transformation.