After logging in, you can change your pixiv profile picture by clicking on Profile Image from Profile below the profile image on your page.
You can also upload animated GIF images for your profile after registering as a pixiv premium member.
* Maximum allowed profile image size is 5MB/image.
Supported image formats for upload are JPG, PNG and GIF.
* In the case of animated GIF, a maximum of 20 frames all formatted to the same display size is allowed.
* Animated GIFs wherein the display size changes between frames is not allowed.
* As of August 21, 2019, it will no longer be possible to upload up to three profile pictures on the desktop and mobile versions of pixiv.
https://www.pixiv.net/info.php?id=5249