For those of you who have been searching without success for the appropriate image sizes for SubscribeStar profiles, the Correct Answers are…
Page Cover: 1240×233
Cover images will display at 1240×233, with a unique aspect ration of 1240:233.
Profile Photo: 380×380*
Essentially any 1:1 image will work (preferably with a higher pixel height and width than 380×380 as scaling up from a lower resolution will reduce quality.
*With the caveat of please allow for bleed, as the cut down on the profile photo from square to “circular-ish” is not as spot-on, despite an image being perfectly centered.