@ss02 It definitely happens for me with the PayPal or Venmo app. I have removed the apps and the file. Then download and install either PayPal or Venmo, no file appears. If I open the app the file appears immediately. No need to actually sign in.
I had a long thread with Venmo support. They claim that their developers have no idea why this file is created. That seems scary for an app but especially so with a financial app.
Part of their response "If you were given the option to create your Venmo account with Facebook and took that option, your Venmo picture will default to using that picture...". But my Venmo is not associated with a Facebook account in any way.
And the .png is not a real PNG nor a backup file. It contains 36 characters.
TL;DR IME definitely tied to Venmo and PayPal apps . But other apps could be causing it.