TOPIC:

I encounterd the fact that in the jem.css about 13 links are made to images that are not present with that same version of jem (download 2.2.1 at 30 july, not 2.2.0 as mentioned current on top of the site)

https://.../media/com_jem/images/menu/icon-16-archive.png
https://.../media/com_jem/images/icon-16-trash.png
https://.../media/com_jem/images/menu/icon-16-default.png
https://.../media/com_jem/images/menu/icon-16-notdefault.png
https://.../media/com_jem/images/admin/checked_out.png
https://.../media/com_jem/images/admin/downarrow.png
https://.../media/com_jem/images/admin/downarrow0.png
https://.../media/com_jem/images/admin/uparrow.png
https://.../media/com_jem/images/admin/uparrow0.png
https://.../media/com_jem/images/admin/publish_g.png
https://.../media/com_jem/images/admin/publish_r.png
https://.../media/com_jem/images/admin/publish_y.png
https://.../media/com_jem/images/admin/icon-16-protected.png

Hi, welcome on our forum where users try to help users.
JEM does not have such links. Some links in JEM are looking for joomla icons:
e.g.
url(../images/menu/icon-16-archive.png);

I don't know how you got these wrong links!

Sorry, but these links are in jem.css. This file is packed in the actual package pkg_jem_v2.2.1.zip
In that file is com_jem.zip. In that package is a folder /media. There is jem.css.
The stylesheet expects these files to be present.
The css starts with:
/**
* @package JEM
* @copyright (C) 2013-2017 joomlaeventmanager.net
* @copyright (C) 2005-2009 Christoph Lukes
* @license www.gnu.org/licenses/gpl-2.0.html GNU/GPL
*/

Near the end of that file you can see following code and more:

#jem .btn-micro i.icon-archive,
.jgrid span.archive {
background-image:url(../images/menu/icon-16-archive.png);
}

#jem .btn-micro i.icon-trash,
.jgrid span.trash {
background-image:url(../images/icon-16-trash.png);
}
....

these links don't go to media/com_jem/images...!
These links take png image-files from joomla!

Strange. The system is looking in:
mydomain.nl/media/com_jem/images/menu/icon-16-archive.png
During a check I get 'not found' error.

The css is in mydomain\media\com_jem\css\jem.css
The url(../images/menu/icon-16-archive.png) also points to mydomain\media\com_jem\images, a subfolder one level above. That is not a Joomla folder. All other png's that are in the package are there.
icon-48-archive.png by example is in that folder.

And, I can not find these 13 files in the entire tree of the most recent Joomla 3. They are NOT Joomla images!

In fact, those files seem to be part of an old Joomla installation. These icons are under:
domain/administrator/templates/bluestork/images/...
and
domain/administrator/templates/hathor/images/...
of an old installation.
These files, png s, are already not in the 10-12-2014 version of bluestork or hathor 29-7-2017.

So, not only the path in jem.css is wrong, they do also not exist in the basic Joomla 3.5 installation.
If these pngs are to be used, they should be included in the package.