When reviewing the history for this BP Add-on, this appears to be a bug that was first reported in April of 2024 in the WordPress support forum for this plugin. However, it appears that the only action taken was the submission of a GitHub issue #112 that acknowledges the bug and this issue has remained open since that time.
Long story shorter, the fatal error is due to, what appears to be, missing parameters which is unforgiving with PHP version 8.0+. Rolling the PHP version back to 7.4.33 produces PHP errors as a warning and notice for the same file and line number. This bug appears to affect the posting of am image in an activity post; meaning the image is uploaded but does not get attached to the associated activity post. What does seem to work is uploading images from a user’s profile media area (where the image gallery is shown).
All I can suggest at this point is that you deactivate the plugin until a remedy can be found and a new version is released.