Your portal logo and the image on the registration page are both uploaded from the app's settings.
Open ConvertOut from your Shopify admin (Apps → ConvertOut), go to the Settings tab, and scroll down to Portal options. Upload your new logo and registration page image there, then click Save.
If an old image is still showing afterwards, hard-refresh the portal page — images are cached by the browser.
Want the same steps with screenshots? See How to change your portal sign-up image and logo.