&bgimage can be used to set the default image avatar, when using &style=0 or &style=6. This only impacts what the person with the parameter added sees and must be either a URL or a base64 data image/svg. URL-encoded values.
If you want to change the background image / avatar on the sender's side for all viewers, use &avatar.
&bgimage2 and &bgimage3
&bgimage2 and &bgimage3 work in conjunction with the existing &bgimage parameter. You pass an URL-encoded image URL to each, and when a guest speaks, it will switch their background image between the 3 possible images, based on their loudness.
eg:
I've included some hand-drawn avatar sample images to test with; they are the default values for &bgimage, &bgimage2, and &bgimage3.
The images will only show when there is no active video and is essentially the same as using &meterstyle=4 with some custom CSS to specify the behaviour, but it is not stream ID specific however.