Sorry for the Reddit reference in the headline, but I need this broken down, please. Is this scenario to be expected?
Is that a Reddit thing? I don't have kids or go on Reddit so let's see how I do. 
If your Marketo-hosted LPs are capable of being loaded over SSL, that means your Marketo instance has a properly installed SSL certificate, which is good.
And it also means your Marketo-hosted images will be capable of being loaded over SSL. That is, if you use https: deliberately in an image URL (src attribute of an <img> tag, or url() style in CSS) , you won't have any errors.
If you start your image URLs with http:, images themselves will still load. But if those http: URLs appear in a page whose main document -- the main doc is the URL in your browser's location bar -- was loaded over https:, you will have a mixed content error.
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