When you select images in Picasa and select this button, the captions of the images will be overwritten by the filename of the file.
Mind: the original caption of the image will be lost!
Updated 2012-02-26:
Filename2Caption now writes the filename to the original Picasa caption location "XMP:description" and also writes it to "IPTC:caption-abstract" for programs like IrfanView that don't recognize XMP tags. To update existing installations of the button, you only need to download and save the script file below.
How to install?
You need to have Picasa installed on your computer, so if this isn't the case yet, download and install it with default settings from picasa.google.com.
Open a "Windows explorer" or "My computer"
Go to C:\
Click the button "New Folder", and if this button isn't available choose "File"/"New"/"Folder" and call it "Tools"
In the folder "C:\Tools", create another one called "PicasaScripts"
Download and save the following files in this new folder ("C:\Tools\PicasaScripts"):
Download script or here
Download ExifTool or here
Install the button in Picasa by clicking the Install button link below and answering yes on the questions... Install button orhere
Note: When the Picasa Configure Buttons screen comes up, if the File2Caption button is in the Available Buttons,
highlight it and click ADD to put it into the current buttons so it is visible in Picasa.