Hi @Ryu ,
Do you want to pass Pen Input image content from your app into your flow, then populate it into the Picture Content in your word template?
Based on the needs that you mentioned, I think the PowerApps could achieve your needs. Firstly, you need to type "Ask in PowerApps" dynamic content within the "TemplateSignature" field of your "Populate a Microsoft Word template" action, to define the parameter which needs to be passed from your canvas app. Then you need to use dataUriToBinary function to convert the previous added parameter value into a binary data.
I have made a test on my side, please consider take a try with the following flow's configuration:

The "TemplateSignature" field in the "Populate a Microsoft Word template" action is set to following expression:
dataUriToBinary(triggerBody()['PopulateaMicrosoftWordtemplate_TemplateSignature'])
App's configuration as below:

Firstly, you need to add a connection to above flow from your canvas app, then set the OnSelect property of the "Next" button to following:
'Your Flow Name'.Run(
Substitute(JSON(PenInput1.Image, JSONFormat.IncludeBinaryData), """", "") // The PenInput image data needs to be passed to your flow
)
The PenInput image is populated properly in my created Word file hosted on OneDrive folder as below:

Please consider take a try with above solution, then check if the issue is solved.
Best regards,