How to open an image file from bash in Mac?
open test.jpg will open the image in the default application for JPEG files (on most Macs this will be Preview) and open -a Pixelmator test.jpg will open in the app Pixelmator if you have it installed. Obviously Pixelmator can be replaced with any application that can open the file.

The open command is quite powerful, it allows for opening files, directories and even URLs. I suggest you read the man page.