Cannot read property 'join' of undefined.
If you select the ‘selectable avatars enabled’ Site Setting, having the ‘allow uploaded avatars’ setting enabled does not allow users to upload custom profile pictures - they have to choose from the list of avatars that have been uploaded to your site. So it is safe to enable these two settings.
JPG, PNG, and GIF files should be fine. The formats that are allowed are set in the ‘authorized extensions’ Site Setting. According to the Selectable avatars topic, you can upload as many as 10 images at a time. The maximum file size for the images is set by the ‘max image size kb’ Site Setting. This defaults to 4096kb. Ideally, you want your avatars to have as small of a file size as possible.
One thing to note is that after uploading avatars and closing the upload modal, you must click the next to the setting to save the list.