When uploading an image to be used as a profile picture, in most situations either through the website or in-app it'll be heavily compressed and the image resolution drops, most notable when looking at your profile picture up close. I've noticed the only way to get around this is as follows.
1.) Go to VRC Website > Gallery > Photos
2.) Click 'Upload Photo'
3.) Upload any 16:9 photo, 1920x1080 might be preferable here.
4.) On 'Preview before upload' screen, make sure 'Set as profile picture' is checked. Upload photo.
5.) Open VRChat and go to your profile, observe your profile picture up-close and notice it's high resolution.
This is the only way to ensure your profile picture is displayed in high resolution, whereas if you were to select another already uploaded image from your gallery, that profile picture would be much lower resolution.