showInfo doesn't work

I'm assuming you're using the Media module.
In order to show info you need to go to "Manage Display" in your content type settings. Select the entity view mode you're using, usually it's "Full Content" or "Default." Make sure the "Format" is Galleria, then click the gear to the right hand side to edit preferences. Make sure the dropdown menu named "Alt Field" is selected with "Alt Text" and the "Title Field" is selected with "Title Text." You're not quite done yet, for the info to actually show up ,there needs to be info stored for the image file. Navigate to your image file that you want to have a brief description of (admin/content/file) and click to edit the file. There you should see a field labeled "Alt Text" and "Title Text." Those are where you type in your image name and description.

It's not documented very well, but Galleria pulls the info from the Alt Text field.

This should solve your problem. Also, if you're using Chrome, try Firefox or some other browser. I currently seem to be having an issue where Chrome won't display the Info when the button is clicked, even on the official http://galleria.io website. It must be a galleria library issue that will hopefully be fixed in the next release.