Was looking for some solution to quickly convert png/gif to jpg that has smaller filesize.
Found this and republish here:

1. Download ImageMagick, the DLL version from ImageMagick Download Page.
Install the program.

2. Start Registry Editor, start > run > regedit.

3. Go to HKEY_CLASSES_ROOT\*\shell
- Right click on shell > New > Key
- Type in: Convert to JPG
- Right click on “Convert to JPG” > New > Key
- Type in: command
- Select “command”. Double click on “(Default)” on your right to change its value.
- Type in with the double quotes: “C:\Program Files\ImageMagick-6.3.4-Q16\mogrify.exe” -format jpg -quality 100 “%1″

That’s all. Simple and straight forward.
I need a few tries before I successfully implement this.

* Take note of the ImageMagick installation path. You may need to modify that. Mine is E:, not C:.
* The double quote (“) before the “C:\” is a MUST. Don’t miss it.
* The folder “ImageMagick-6.3.4-Q16″ might be different depends on the version you downloaded.