A few thoughts on this in the spirit of giving feedback to others trying to be entrepreneurs.

- Looks like it could be useful for some people who want more than just a basic screen shot.

- Looks like it's probably easy to use. The apparent focus in the marketing copy at least on having a nice workflow is encouraging, that the developer is thinking about the right things.

- It's a bit odd that sitting here in the US, when I click on the link on the site leading to the App Store web page, it shows me a UK price. Is this only in the UK app store?

- You're submitting a few times, which is explicitly allowed here as far as I remember. I wonder if the lack of traction is just that people don't ship apps that often? In other words it's not something each and every person does every day.

- That price, WOW. A bit high, for pasting together some images and text and exporting to multiple resolutions. Those are not rocket science. I'm almost angry and hesitant to share my feedback just due to the price alone, but I'll write it off as a mistake.

- Not sure if you've thought through how exporting to multiple resolutions helps for apps where the UI looks different on different screen sizes. In the era of auto layout and size classes, not every app will have screens that scale identically on multiple iPhones.

- Taking screen shots the right way (using the Simulator, so a real world carrier name doesn't show up, and so the battery is always full without a second thought) is so dead easy -- Command-S -- that an entire large slice of the possible market pie just isn't there, because their existing solution is so dead simple. They can do one (combo) keypress for each screen, drag-drop the group of files onto iTunes connect, click the tab for the next screen size, and repeat.. couldn't be faster. I did this task about 10 days ago for a Universal app and not counting the build time for each device size, which you would have to incur anyway, it took maybe 20 seconds per set of all screenshots (I had four) per device size, and that includes launching the app and navigating to each screen I needed to capture. Not exactly an onerous task.

- I wouldn't call what you have screenshots; they are marketing images that include screenshots, which are uploaded to the screenshot slot in iTunesConnect, at least in the case of the iOS platform. I couldn't speak to Android.

- If I had to make some spiffy marketing images (not screenshots) then I would consider a tool like this at a much lower price point, like $5. Given no such tool, I would just use GIMP then convert the sizes with ImageMagick convert. A lot of iOS developers probably already have a script that does that too.

To end on a positive note I think you should focus on the marketing images aspect of this (as opposed to screenshots) because screenshots are so easy as is, you need to differentiate yourself. One possible direction, but more work for you, would be to build out other formats and image sizes to allow the user to sit down and in one session target the App Store, Facebook ads, Twitter ads, Google ads, etc. creating all the images needed and guiding them in text length limits for all the requisite fields. Going beyond that you could also get into creating the related web assets for their web presence, all based off of whatever common theme, text, and images they start with.