HubSpot's blogging platform was designed to make managing your website's SEO easy. One of the ways our system does this is by checking blog content for the presence of appropriate alt tags on any images inserted into a post. If you've inserted an image into your post that has an empty alt="" attribute in the html this warning will be generated.
In the COS Blog you can insert alt-text to an image by clicking the image, "Edit Image," and insert your description in the "Image Description" section.
If you are using HubSpot's CTA tools within a blog you may also see this warning. HubSpot's CTA buttons are generated through a string of css into the post body content. This can cause a conflict with the flag checker on CMS blogs and prompt this error to appear. If you receive this error verify that all images contain alt text and then proceed with publishing your post. The CTA's alt text will be generated from the tool when the page loads on the live version.