Hi @ericonline ,
Please refer to the screenshot of the flow. Assuming that you already created the powerapp and the button control triggers the flow.
Expression on the button
'PowerAppsbutton-2'.Run("TestYash.jpeg",Camera2.Photo)Here, PowerAppsbutton-2 is the name of my Flow, TestYash.jpeg is the file name I am requesting to powerapps from flow, Camera2 is the camera control and Camera2.Photo is the blob I am sending from powerapps to Flow. 
Expression in compose:
dataUriToBinary(triggerBody()['Initializevariable_Value'])
Hope this Helps!
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