Be aware, in Europe most commercial companies won't go near MS Publisher! I don't know if it is the same in the States, so check first.

Here is my final method, making 6 rows instead of three. I have not gone to a lot of trouble and the workmanship is terrible, but as you have not come back on anything I have said so far, everything I do may be falling on stony ground.

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Make sure the images you want to print are in the film strip
In the Print Module, first goto Page Setup and choose the paper, size etc.
Choose Custom Package, a plain white sheet will show, and if you press "i", some info will appear on the screen.
Set the toolbar for Use "Selected Photos" but do not select the images required for the Collage.
Click and drag the first image you want and place it onto the white screen, resize it either by dragging the image handles or by altering the sizes in the "Cells" panel.
Arrange the images how you want them by clicking and dragging them. You can have all different sizes or all the same size.
If you need a second page, click the New Page button.
An image is considered selected when it shows the grab handles.
the Rules Grids and Guides panel is there if you want to use it. (On screen only, does not print).

This image is only 2340 x 1560 pixels - Have you already cropped to 18 x 12"?

If there is more image available on the sides it may be possible to do something. At present, as you are aware, the finished print will simply be a long thin "panorama" type strip if the ground works and top foliage is simply cropped.

So far, I have suggested two methods: 1. Stretching the the image top to bottom, obviously a great big no no.
2. cropping the image and building a frame - end result is the panorama strip

A third method would be to use whatever extra you have on the sides as part of the image, crop that to 18", but of course the down side will be the "posers" will be smaller.

Don't forget that if you choose the frame, a captions can be written under the photo "Staff of the Acme Company - August 2016"

Here is a crude example of this. The white area is 18 X 12".

Sorry about the quality but your download did not have enough pixels.

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Your image is already at the correct ratio to print at 18 x 12". A quick and dirty method of cropping the foreground and background and still maintaining the aspect ratio, is to distort the image. I doubt this is what you want, but I have attached a copy vertically distorted.

A better method of cropping this one would be to crop to smaller than 18 x 12" and place a white or tinted border around to fill the 18 x 12" space.

It can be done (on Windows - no idea about Macs) but should not be done. Lightroom is not designed for one offs, and can leave the user tearing his hair out because of the mess that can be caused by even trying to open it with the "Open With" procedure.

If an image is opened in LR using "Open With", the import dialogue will open and to actually do anything with the image, the normal import procedure must be followed. The image is catalogued and any processing is saved in LR*. When done, there is no Save or Save As, the file must be either exported or left alone. If exported, there will be two different images now on the hard drive (possibly with the same file name), if left alone, there will be the original image on the hard drive and a different version when viewed in Lightroom.

So far, not a problem, but what happens when you reopen the same image with the Open With command? LR may refuse to open the photo if the file name is unchanged, if the preferences in the Import dialogue allows duplicates, you could then end up with three images on the hard drive, all same file name, all different derivations of the same photo.

If you are using LR, use it as the designers intended. Better results and less hair loss.

* DNGs have the LR processing saved both in the catalogue and in the file itself. Raw files may have a xmp sidecar file with all the processing info with it. Not sure if the Open With would carry the xmp file into LR with it.

Bridge does not have any photos in it. They are all on your hard drive. In PS goto File > Open and navigate to the file you want to work on. When done Save or Save as (new name)

Reboot computer, goto Creative Cloud app and sign out, now uninstall Bridge. Go to Creative Cloud, sign in and install Bridge.

In the import panel, hover your curser over one of the images and make a note of the information which shows up on the yellow tips tag:(File name, date and time).

Now open the Library > All Photographs

In the search box Choose the Text option, Any Searchable Field: Contains All: (Now enter the file name)

Assuming the image shows up, right click on that image and choose "Go to Folder in Library"

You should now have found the image already imported.

It is possible that you have another image with the same file name, by following the above that image will show up instead or as well as.

Penultimate? Comes up quite often in Legal documents - it is also the name of an iPad application by Evernote for capturing hand writing on a tablet.

Antepenultimate: I am going to have to use this ten times and add it to my vocabulary.

The nearest I can think of is to sort by Capture date (set up a smart collection), but this will not work if the desired import contains more than one date, or of course if the capture date bears little relationship to the import date.

Are you referring to an iPad program?

Thinking that you probably do not use Photoshop, I searched and found a tutorial for making the flag in GIMP.
http://www.artistsguidetogimp.com/?page_id=357

Like this?

Photoshop using the Distort >Wave filter

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