Is it okay to edit the webform content type?
I don't think you've necessarily done anything wrong by adding fields to the Webform content type, but if you're interested in strictly keeping your code separate from core code, you can create a new content type,  and enable Webforms to be used on this content type under "Webform-enabled content types" on /admin/config/content/webform.

Then you'll be able to add your own image field to your own content type without problems.  This will then leave you free to export the content type configuration to code with the features module, without really interfering with the Webform content type provided by the Webform module.