You can use[rwmb_meta]shortcode built in Meta Box plugin to deal with the problem of inserting values of custom fields in post content without opening theme files and inserting PHP code.

The shortcode take the following attributes:

[rwmb_meta post_id="15" meta_key="field_id" type="image" size="thumbnail" ...]

The [rwmb_meta] shortcode works exactly like rwmb_meta function (look at this documentation). All attributes are the same.

#Shortcode attributes

post_idThe post ID. Optional. If not defined, then the ID of current post is used.
meta_keyThe meta key, same as field ID. Required.
typeField type. Default is text. You can bypass this attribute if your field has single value (e.g. not multiple, not clone). Required for field which is multiple or clone (checkbox_list, file and image fields, taxonomy, etc.).
multipleIf field has multiple values (like select with multiple selections), set this argument to true. It’s automatically set to true for checkbox_list, file and image fields.
sizeImage size, used for image fields only. If not present, the thumbnail size will be used.
taxonomyThe taxonomy for which to retrieve terms, used for taxonomy fields.