Take a look at HELP! How to link Kobo server to ODK Collect on Android Tablet
Adding images to select options in the Kobo KPI formbuilder UI is probably not going to be a trivial development task, especially since you are now having to handle media files alongside the raw form data. I’d probably recommend opening a feature request - with any luck maybe its already on the Kobo roadmap… [that is, nope I cant give you directions how you might go about implementing it yourself, sorry]
