You can either set the image as an extrafield.
This has its drawbacks since the template has to be updated, or you can search for a 3rd party extension at the JED/KED which extends the item's form in the backend.
If you are strong with JS you can download the example plugin (
getk2.org/extend/extensions/90-example-k2-plugin-for-developers ) and use it to pass a JS/jQuery script which checks if the image field is empty and allows to proceed.