How to attach an image in the middle of comment

The ability to paste images in Zendesk will depend on what settings an Admin has configured within your account. This help article should be helpful for you: https://support.zendesk.com/hc/en-us/articles/228604408-How-do-I-add-images-into-the-body-of-a-ticket-

I paste images in-line in Zendesk often and either use the method you're describing or drag the file from Finder (on a Mac) to drop the image in-line. I have a feeling you may be seeing "null" due to some ticket settings. Hope that helps point you in the right direction!