For those who are still working on generating a signature with Flow and Powerapps I created a video that should help.
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I've been dealing with the same problem recently. I followed this tutorial that explains how to display attachments in a gallery (it can be images for example) https://www.youtube.com/watch?v=e32WHs53HLo .Then, when I got the same result, I wrote in my HTML code in PowerApps : <img src='" & GalleryDisplayingPictures.Selected.AbsoluteUri & "'>That's all ! Not perfect because you need to select the gallery's item first, but it's working.
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Features releasing from October 2019 through March 2020