The hero image you use for your interactive is the main star of the interactive. It's the first thing visitors will see. It's super important it's as good as it can be so here are some tips:
It's best to use .jpg files and to make sure they are large in file size - a few Megabytes is a good sign (up to 10Mb). That means visitors will be able to zoom in on the detail of the image.
Make sure it's a good quality image - if it's a single object it should be well lit and just contain the object itself. (You could always crop the image to help that)
It's probably best if it doesn't have a border or anything around it, just the image itself.