How do you change thumbnail pictures when you hotswap
Do the usual for setting an image with an avatar.
Import an image you want to use as the image, select it in the project tab, set the texture type to "Sprite (2D and UI)" in the inspector tab, then hit apply.
Then, put it into the scene by dragging the image from project tab to the hierarchy tab. Flip the image along the Y axis (Rotation > Y) to 180 to flip it the right way.
On the SDK, build the avatar and go through the hotswap process. On the name and description screen after (the usual VRCSDK screen) the camera should be adjusted to fit the added image to the frame (select the "VRCCam" item in the hierarchy tab, then move it using the position controls in the Inspector tab). Upload as usual.
Just tested as I write this, so it should work for anyone.