If you have troubles to upload pictures to your gallery or changing your profile picture, please open the Settings menu of your device and check if you have still enabled permissions for the access of the app to your camera and gallery.

More information for Android users:

On your phone, open the Settings app.
Tap Apps.
Tap the app you want to change. If you can't find it, tap See all apps.
Tap Permissions. If you allowed or denied any permissions for the app, you'll find them here.
To change a permission setting, tap it, then choose Allow or Don't allow.

More information for iOS users:

To find Permissions, open up the Apple menu, then choose System Preferences. From there, click Security & Privacy, then open the Privacy tab. Here you can see all the permission categories, from location to app analytics. Click on any of the entries on the left side to see which apps have requested and been given permission.