You can still keep the If statement since it’ll just make sure to only update the value if there is something to update with. Otherwise you’ll end up with no image if you do an update and haven’t set a new image in the form.
Also make sure your field is truly named “post_image”. Or change it to go by the field key which is actually prefered since it’ll retain the functionality even if you change the field name later on.
acf/update_value/key={$field_key}
You can find the field key by heading to screen options (top right) in acf admin and checking show field keys.
