A Media (OBJE is the corresponding tag) is an entity wrapping any
multimeda data you can think of.
GenJ is aware about image files and will try to display the
information in any of the views.
When creating a media by clicking on
you can specify a file by opening a file-chooser. The data of that file
is included in your Gedcom-file (Base64 encoded). To keep the file
small it is also possible not to choose a file at that point. Simply
add a FILE property to the entity later that points to a file on disk.
This is an external file that has to be shipped separately.
Medias are by default not linked to any other entity. After creation you can add a OBJE property (yes, OBJE is the tag for a media entity and the tag for the media property) to any entity and link it to a media by entering its id and pressing link.
A media according to Gedcom has default properties - here are two examples based on the inclusion of the data or a reference to an external file.
@M004@ OBJE TITL Taken from myimage.gif FORM gif BLOB 585 Raw Bytes
@M005@ OBJE TITL Pointing to local file FORM jpg FILE myimage.jpg