It is often appropriate to start by describing the most striking abnormality. However, once you have done this, it is vital to continue checking the rest of the image. Remember that the most obvious abnormality may not be the most clinically important.
In an exam, don't make the mistake of spending too long going through a rigid systematic approach while ignoring obvious abnormalities.
The rule is - don't ignore the 'elephant' on the image - describe its long trunk, its big ears, its tusks, and its rough, grey skin. Once you've done this you stand a better chance of diagnosing the 'animal' you are dealing with, but you should then continue by using a systematic approach to look at the rest of the image.