Just for fun, I thought I'd try to build an image-to-ASCII-art converter in JS.
To use it, you'll need to unzip the archive, and place a GIF, JPG or PNG file in the same directory as the HTM file.
Next open the program under Firefox or IE9 and type the complete file-name of the image into the form.
Then press the Generate button. If the file-name is correct, the image should be converted into ASCII art.
It's only been successfully tested under Firefox and IE9. Chrome doesn't work due to a false-positive in its default security settings.
(edit) I have improved the code for better contrast handling, but only slightly.
Here's an output sample:
¤xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx•
¤xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx•
¤xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx•
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx·
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx·
xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx·
NNNNNNNNNNNNNNNNNNX¤¤¤¤¤¤¤¤¤¤¤¤NMMMMMX¤¤¤¤¤•············
MMMMMMMMMMMMMMMMMMN••••••••••••NMMMMMX••••••
MMMMMMMMMMMMMMMMMMN••••••••••••NMMMMMX••••••
·xxxxxxxxxxxxNMMMMMXxxxxxx••••••••••••NMMMMMX••••••............·
.MMMMMM••••••NMMMMMX••••••••••••••••••NMMMMMX••••••••••••••••••·
.MMMMMM••••••NMMMMMX••••••••••••••••••NMMMMMX••••••••••••••••••·
.MMMMMM••••••NMMMMMNxxxxxx••••••••••••xXXXXXXXXXXXx••••••••••••......·
.MMMMMM••••••NMMMMMMMMMMMN••••••••••••••••••XMMMMMN••••••••••••••••••.
.MMMMMM••••••NMMMMMMMMMMMN••••••••••••••••••XMMMMMN••••••••••••••••••.
·xxxxxxxxxxxxXXXXXXXXXXXXx••••••••••••xxxxxxNMMMMMMxxxxxxxxxxxx•.....·
MMMMMM¤••••••••••••••••••••••••NMMMMMMMMMMMMMMMMMMMMMMMM.
MMMMMM¤••••••••••••••••••••••••NMMMMMMMMMMMMMMMMMMMMMMMM.
XXXXXX¤•••••¤•••••¤••••••••••••XNNNNNNNNNNNNXXXXXXXXXXXX.
••••••••••••••••••••••••••••••••••••••
••••••••••••••••••••••••••••••••••••••
· •¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤¤• ·
xxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxx
xxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxx
xxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxx
·xxxxxxxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxxxxxxx·
·xxxxxxxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxxxxxxx·
·xxxxxxxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxXXXXXXXxxxxxxxxxxxxxxxxxx·
.xxxxxxxxxxxxxxxxxxxxxxxxXXXXXXXXXXXXXXXXXXXXXXXXXXxxxxxxxxxxxxxxxxxxxxxxxx.
•xxxxxxxxxxxxxxxxxxxxxxxxXXXXXXXXXXXXXXXXXXXXXXXXXXxxxxxxxxxxxxxxxxxxxxxxxx•
•xxxxxxxxxxxxxxxxxxxxxxxxXXXXXXXXXXXXXXXXXXXXXXXXXXxxxxxxxxxxxxxxxxxxxxxxxx•
.¤¤¤¤¤¤¤¤¤¤¤¤xxxxxxXXXXXXx¤¤¤¤¤¤XXXXXXXXXXXX¤¤¤¤¤¤xXXXXXXxxxxxx¤¤¤¤¤¤¤¤¤¤¤¤.
.••••••••••••xxxxxxXXXXXX¤•••••¤XXXXXXXXXXXX¤•••••¤XXXXXXxxxxxx••••••••••••.
.••••••••••••xxxxxxXXXXXX¤•••••¤XXXXXXXXXXXX¤•••••¤XXXXXXxxxxxx••••••••••••.
.•••••¤••••••¤¤¤¤¤¤xXXXXXxxxxxxxXXXXXXXXXXXXxxxxxxxXXXXXx¤¤¤¤¤¤••••••¤•••••.
.••••••••••••••••••xXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXx••••••••••••••••••.
.••••••••••••••••••xXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXx••••••••••••••••••.
.•••••¤••••••xxxxxxXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXxxxxxx••••••¤•••••.
.••••••••••••XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX••••••••••••.
.••••••••••••XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX••••••••••••.
·............XXXXXXXXXXXXXXXXXXx¤¤¤¤¤¤¤¤¤¤¤¤xXXXXXXXXXXXXXXXXXX............·
XXXXXXXXXXXXXXXXXX¤ ¤XXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXX¤ ¤XXXXXXXXXXXXXXXXXX
......XXXXXXXXXXXXx¤¤¤¤¤• •¤¤¤¤¤xXXXXXXXXXXXX......
.MMMMMMMMMMMMMMMMMM¤ ¤MMMMMMMMMMMMMMMMMM.
.MMMMMMMMMMMMMMMMMM¤ ¤MMMMMMMMMMMMMMMMMM.
······•MMMMMMMMMMMMMMMMMM¤ ¤MMMMMMMMMMMMMMMMMM•······
¤MMMMMMMMMMMMMMMMMMMMMMMM¤ ¤MMMMMMMMMMMMMMMMMMMMMMMM¤
¤MMMMMMMMMMMMMMMMMMMMMMMM¤ ¤MMMMMMMMMMMMMMMMMMMMMMMM¤
¤MMMMMMMMMMMMMMMMMMMMMMMM¤ ¤MMMMMMMMMMMMMMMMMMMMMMMM¤
I may need to make some adjustments.
(edit) I've updated again - I should really do more testing before I release

Charset 10 is now the default, non-existent files are handled better, and you can now choose between 3 scaling options. Also, the courier font aspect ratio is better catered for.