I took this image analysis example from the site of Able Image Analyzer. It was easy to reproduce the results with Pixcavator.

Setting the contrast slider at 100 gives you good contours for both Sicily and the calibration bar. The length of the latter is see to be 217 pixels, so

 217 pixels  = 250 km.

The are of Sicily is see to be 18884 pixels, so

 the area = 18884*(250/217)^2 = 25064 sq km.

(Here 250/217 is the calibration factor). The actual area is 25706, so the error is about 2.5%.

 See other image analysis examples.