For example:
It is necessary for you that all photos in galleries of "participants" would be in jpeg. But different types are allowed for loading, and all of them will be cut off in compliance with the type.
I suggest to choose expansion in which the reduced copies of the image will be converted, it can be both control for a field, and expansion for API Image
$image->ext() //jpg
$newImage = $image->ext('png')
$newImage->ext() //canonical png convert