You do not need to put anything specific in the .cdsplotinit to save to png or jpg. You just have to have a .cdsplotinit existing - as I said, this is a rather odd need, but it's because the OCEAN interface to producing an image is performed via the printing API - even though it doesn't actually using the printing capability within ViVA at all.
We are considering separating the API to save images from the hardCopy() function - the problem is that we're overloading both printing and saving of images into a single API, which over-complicates things. This is a historical decision, but with the increased flexibility we have in IC615 for saving images, it is making less sense now.
So, if you use the .cdsplotinit I provided above - or indeed any .cdsplotinit - you will be able to save images in any format using the SKILL API.