If you are using a plugin such as simple_captcha that requires RMagick, then you may encounter the following error during the rendering of jpeg catpcha image on Linux: (seen in the logs)

ArgumentError (unknown format: JPG):
/vendor/plugins/simple_captcha/lib/simple_captcha_image.rb:95:in `format=’

This mean that we need to install the delegate library for JPEG.
1. do apt-get install libjpeg62 (not sure if this is required or just dev is enough)
2. do apt-get install libjpeg62-dev
3. cd to where you downloaded Imagemagick and do ./configure, make, make install.
The result of ./configure will summarize the status of deletgate libraries in the end.

After this, a blank image should start to show up in place of the simple_catpcha image and the logs may show something like this:

sh: gs: command not found
sh: gs: command not found
RMagick: unable to read font `(null)’.
RMagick: Postscript delegate failed `/tmp/magick-XXfm5AzA’: No such file or directory.
sh: gs: command not found
sh: gs: command not found
RMagick: unable to read font `(null)’.
RMagick: Postscript delegate failed `/tmp/magick-XXg7z0RL’: No such file or directory.

4. Do apt-get install ghostscript if simple_captcha image shows up as a white blank image, without any letters. Don’t forget to do ./configure, make, make install from your Imagemagick source dir after this. Restarting of the server will be required for production environment.

Example result of ./configure status.

Delegate Configuration:
….
JPEG v1 –with-jpeg=yes yes
FreeType –with-freetype=yes yes
…

These 4 delegate libraries got simple_captcha working.
1. libfreetype6-dev (This also seems to be for fonts, but didn’t cut it for simple_captcha, but I had installed it already, so I let it be.)
2. libjpeg62-dev
3. libpng12-dev ( installed it just in case…)
4. ghostscript
Running ./configure, make, make install from imagemagick source dir is necessary after installing the above libraries.

If captcha image is not getting recognized, you may need to run the following (for rails > 2.0, there is a separate command for older Rails which can be seen from the simple_captcha website.)
rake simple_captcha:setup

then run
rake db:migrate