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to change default cover image (events, profile, groups) you can go to \components\com_community\assets
you will find 5 covers there :
cover-event.png, cover-female-default.png, cover-group-default.png, cover-male-default.png, cover-undefined-default.png
you can overwrite this 5 files with your own... please make sure the dimensions must be same and the file name too..
if you want change only for profile, you can overwrite this 2 files : cover-female-default.png and cover-male-default.png..
But, someday if you did an upgrade, this covers will be replaced with jomsocial original image again.. so make sure you had the copy of your cover image on your computer..