You can insert it the same way you do on any other page. The thing you cannot do with a template is attach an image to it. The image will have to be attached elsewhere in your wiki. I would recommend a section that acts as a resource library for these types of things.
You can put an image in a template. You just can't attach it to the template. The image must be attached to an existing page. I typically recommend having a page per space that works as a document repository. Any time you need to reference the document you link to the library. That way you only have one version. So, in your scenario you would attach the logo to an existing page (document library) and then add the logo to your template. You can add images to a page that are attached to a different page.
It looks like the only workaround right now would be to create a page and put the logo on it. Then you can add either a page include or excerpt include macro to the template to pull in the image. Not ideal or friendly, I know, but it is the only workaround that I know of right now.
I just submitted a similar enquiry. Interestingly this has been flagged a year ago and the issue still persists. I agree that this is a very bad experience in an editor that allows so many other things to be inserted in a template except a simple picture. It does not make sense to me.
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