The image captcha module works best with a TrueType font file (TTF) for generating the images, but because of licencing issues it is not allowed to package fonts with the module. A simple built-in bitmap font is provided as default instead. You can install TrueType fonts yourself by putting them in the fonts directory of the image captcha module (directory "%fontsdir") or by uploading them to your Drupal file system (directory "%filesdir") with for example the upload module.