If the image you have opened has flat colors, poor contrast or is lacking in detail you can use the enhance tools to significantly improve the image.
Typically you can use the Levels tool to adjust the dynamic range of the image.
If you prefer you can use the Brightness/Contrast/Gamma tool to individually adjust the Brightness, Contrast and Gamma in an image.
The enhance tools allows you to bring out more detail in the image. As a result enhanced images are often less compressible than their originals. This may be the limiting factor on how much enhancement you can apply.
If possible you should use an uncompressed or high quality JPEG image to avoiding amplifying the effects of compression artefacts.