This is a known issue. The Avatar feature is not compatible with a number of mobile browsers. Even if it is compatible with yours, you may be running into a more common issue which is file size. Avatars must be no larger than 500 KB. If you are trying to add an image that was taken by your mobile device, it is likely way larger than 500 KB and thus not valid for use as an avatar.
The limits and recommendations on images you can use are listed at Help:Avatars: Your image must be a JPEG, PNG, or GIF, and it must be less than 500kb in size. It's best that the image is 150x150 pixels or larger.