Unfortunately, the Picasa template does not have an interface which allows you to change configuration options so you'll need to set the options manually.

After you have created the gallery, you can open the gallery's 'config.xml' file in a plain text editor and add your options to the file, as attributes to the opening <juiceboxgallery> tag, e.g.:

imageScaleMode="SCALE"

Alternatively, you could open the plugin's 'header.xml' file in a plain text editor and add your options to the file (as before) and all the options listed in the file will appear in all galleries created by the plugin.

(You could also open a gallery created by the Picasa template in JuiceboxBuilder-Pro to tweak the settings on the 'Customize' tab before re-saving the gallery on the'Publish' tab.)

For reference, more information about setting configuration options can be found here.