What is the crowd's opinion of
Magic Mockups?
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2 reviews of this app found across the web:
A quick google will bring up plenty of psd templates to make your own or you could use MagicMockups. Its a great little site where you can select a template, upload an image of your site or give it a url and it displays your site in a professional looking mockup. Just resize/crop the created image to fit Instagram dimensions.
If you want still life photos of products in use, https://placeit.net is/was great - you just drag and drop an image onto the webpage and it generates the mock-up for you. However, it used to be free, but now you can only get a quite small image for free.
A bit of research shows these people are now doing the same thing, for free and with no attribution required, although with far fewer photos to choose from: http://magicmockups.com/
EDIT: Oh, and another one here: http://mockdrop.io/ Seems like a few people are doing this now