You can now choose the file-type for the files you save using Screengrab! Your choice is limited to JPEG or PNG, but at least there’s a choice.

The default is configurable in the Preferences page, but it can also be changed whenever you are saving a file.

The JPEG quality is not yet configurable in the GUI, but it’s default is about 75%.

How?
Enter the Screengrab! Settings page, using the Add-ons window and select PNG or JPEG on the “Image Format” radio.