In addition to the image formats you mentioned above (PNG, JPG, JPEG, GIF), as far as I know, several other image formats can be used here, e.g., BMP and APNG.
Support for the WEBP format has been proposed on the Github of SimpleMachines (https://github.com/SimpleMachines/SMF2.1/pull/6271), but it seems that it cannot be realized if you look at the discussion at the following link: https://github.com/SimpleMachines/SMF/issues/6545.
I don't know exactly how urgent the WEBP format is to be implemented on Bitcointalk, considering that this forum still uses an old SMF version (1.1.19).