As I didn't know where this thread went to since the wiki was up again, and therefore had no idea what is going on, I already uploaded an image on Jan 31 which showed up in an old version in the wiki page. That image's filename is http://wiki.flightgear.org/File:EC130-B4_MAC_rescue_equipment_2.jpg uploaded again on Jan 31, 2015
The problem with the missing thumbnail of today is related to the following code in the wiki page of EC130 B4 (link to page: http://wiki.flightgear.org/Eurocopter_EC130_B4:
[[File:EC130-B4 wide luggage box-012.jpg|thumb|ZS-HOY of Starlite Helicopters with wide luggage boxes seen from above]]
If I click the enlarge icon beside the empty space with the missing thumbnail, I get the image displayed. So it seems it is not really lost

The link of the image displayed is: http://wiki.flightgear.org/File:EC130-B4_wide_luggage_box-012.jpg
Do you see why I talk about a naming issue related to Blanks and underscores ?!
It seems to look for an image with spaces in the name and the image file has underscores in the name instead.
B.T.W.: I don't know who removed the underscores, because the original file I uploaded months ago, had the underscores in the name. The usual procedure I followed was to use the upload wizard and then use the link it displayed after successful upload in the wiki page. Remark: This tool has changed now too and doesn't show the links as it did before.
Additionally I had copied these links I got from the upload wizard in a text file I keep in git. There the original link of the above lost image is as follows:
[[File:EC130-B4 wide luggage box-012.jpg|thumb|EC130-B4 wide luggage boxes seen from above]]
So my theory: The old Wiki software used to remove the underscores, as the file had underscores and the link presented after upload didn't have the underscores.
Before the crash or Wiki upgrade this worked well.
So far I didn't experience problems with other images on the page although several others are listed on the "lost" page ...
Not being a Wiki insider this is all I can contribute it seems.
Thanks for working on this issue.
Mike-DE