When an asset is opened, the details about it are displayed. For images, there is a feature for directly comparing different versions. If several versions of a picture are available, the comparison slider can be activated via the icon in the asset header:
In the thumbnails of the versions you can also choose which version should be displayed on the left (green L) and which version on the right (green R).
Now you can use the mouse to move the yellow slider in the middle to the left or right to compare the two versions.
By clicking on the 
button, the version comparison is ended. If you wish to save your new version click on "Save & Close" at the bottom.