If you are using Windows, you will need to download and install ImageMagick. ImageMagick is a free third party tool that handles the conversion of many different file formats. Its used to convert the images to small thumbnails.
I generally get the top link via HTTP.
The Command Line should start with just "convert.exe" and not "./convert".
The Environment Variables field should be blanked out for Windows too.
If you want to make thumbnails of PDF or TIFF images, then you will also need GhostScript installed. Click on the latest release.
Then click on either the 32bit or 64bit version.
Other ImageMagick Installations
If ImageMagick is installed on the OS, and the command "convert" functions from a terminal, do the following to configure CrushFTP to use it.
Do a "which convert" to see the folder where ImageMagick is installed. In my case, it was /opt/local/bin/.
Set the Working Directory to be this path: /opt/local/bin/
Set the start of the Command Line to be "./convert "....
Next do a "echo $PATH" to see what the PATH is in your terminal.
Copy this, and paste this for the environment variables as follows:
PATH=(pasted PATH info you copied)
Enable the debug checkbox, and let it run to see the results. (Restart CrushFTP to make it run immediately at startup.)