Here's a picture:

Dark, isn't it?
Here's how to make it lighter and brighter using IrfanView.

( Download IrfanView for Free from :
http://www.irfanview.com/ )
To begin just load the dark pic you want to fix into IrfanView.

Now go to the "Image" menu and choose "Enhance Colors..."

You will see the Enhance Colors controls.

DO NOT TOUCH THE "Brightness" CONTROL! Ignore it completely. You will not need it.

Start fixing your photo by INCREASING the "Gamma Correction" value.

Either click on "OK" or "Apply to original" to see how your photo has been lightened.

Maybe too much? Too much is what you want!

Adjusting the Contrast will fix this!
So INCREASE the "Contrast" value until your picture looks about right.

Then click "OK" or "Apply to original" and have a look at how your fixing is progressing.

Of course different values give different result so try out different combinations -

- untill you happy with they way your picture looks.

Now, increasing the contrast can make the colours of the photo too bright and intense.
If this happens just DECREASE the "Saturation" value to reduce the intensity of the colours in the photo.

And that's it! Your picture is fixed!

Save your picture using "Save as..."

Give it a new "File name" and click "Save".

Now you have the original version and the fixed version to compare!
Before:

After:

X!