Did you try it on a fresh project created within your Webflow Dashboard?
The preview link you’re showing in your screenshot is a read-only project, meaning it won’t let you make certain changes before actually purchasing the template. This feature is mainly so you can see how the template is built within the Designer before making your purchase, however editing this will be limited in a number of ways.
Assuming you’ve created an account, head into your dashboard and click the large blue “+ New Project” button in the upper right. I’d recommend selecting the Blank template as this won’t be something you’ll continue with (it’s only to test the image upload), however you have a number of free options to choose from on the template selection page.
I’d imagine that you shouldn’t have any issue uploading the photo to this new project—in which case you shouldn’t have a problem uploading/changing images once you purchase the template in question.