How do I resize an image which takes into account correct aspect ratio?

In Flow image resize always takes aspect ratio into account unless you go out of your way to force weird sizes. 

In Claro if you use the Control Bar inputs to resize, check the box to maintain aspect (default is checked). If you resize on the Stage, click the bottom corner and hold Shift to lock aspect ratio. 

If your image looks off, you can reset it to match the Asset size from the Context Tab when the image is selected. Once reset, resize using the noted aspect control methods.  Reset only applies to Claro.