Only one image displayed in galleries.

Yes. Is views optional?

The paging settings are at the default values e.g. 12 entries per page and show pager is set to yes.

When I test the view within the views section it also only shows one picture. However if I execute the printed sql query

SELECT node.nid AS nid, 
node.title AS node_title, 
node.type AS node_type, 
image.fid AS image_fid, 
image.image_size AS image_image_size, 
node.changed AS node_changed 
FROM node node 
INNER JOIN term_node term_node ON node.vid = term_node.vid 
LEFT JOIN image image ON node.nid = image.nid 
WHERE (node.type in ('image')) AND (term_node.tid = 9) 
ORDER BY node_title ASC, node_changed DESC

it gives the expected result:

+-----+-------------------------+-----------+-----------+------------------+--------------+
| nid | node_title              | node_type | image_fid | image_image_size | node_changed |
+-----+-------------------------+-----------+-----------+------------------+--------------+
|  15 | Flyer: Feuerzangenbowle | image     |        23 | _original        |   1282295252 |
|  15 | Flyer: Feuerzangenbowle | image     |        24 | thumbnail        |   1282295252 |
|  15 | Flyer: Feuerzangenbowle | image     |        25 | preview          |   1282295252 |
|  15 | Flyer: Feuerzangenbowle | image     |        26 | inline           |   1282295252 |
|  21 | Testbild                | image     |        31 | inline           |   1284104088 |
|  21 | Testbild                | image     |        28 | _original        |   1284104088 |
|  21 | Testbild                | image     |        29 | thumbnail        |   1284104088 |
|  21 | Testbild                | image     |        30 | preview          |   1284104088 |
|  13 | Weltmeisterschaft 2006  | image     |        10 | _original        |   1282294473 |
|  13 | Weltmeisterschaft 2006  | image     |        13 | thumbnail        |   1282294473 |
|  13 | Weltmeisterschaft 2006  | image     |        14 | preview          |   1282294473 |
|  13 | Weltmeisterschaft 2006  | image     |        15 | inline           |   1282294473 |
+-----+-------------------------+-----------+-----------+------------------+--------------+
12 rows in set (0.00 sec)

If I change the design field from "gallery" to "html list" all pictures are shown correctly so I guess this issue is not related to views but to the views part of the image module e.g. the design or something.

I made no changes. However if I remove the "image" field from the view I get the correct query and the correct results but no images. :-|

Just tried with a clean install. Enabled views, image, image-attach and image-gallery modules, created a gallery and uploaded 2 pictures. The same behaviour as described the thumbnail of the latest upload is shown in the overview page but only one image (the first uploaded) inside the gallery.
When going to admin/build/views/edit/image_gallery and checking the sql query I get the following:

SELECT node.nid AS nid, 
node.title AS node_title, 
node.type AS node_type, 
image.fid AS image_fid, 
image.image_size AS image_image_size, 
node.changed AS node_changed 
FROM node node 
INNER JOIN term_node term_node ON node.vid = term_node.vid 
LEFT JOIN image image ON node.nid = image.nid 
WHERE (node.status <> 0) AND (node.type in ('image')) AND (term_node.tid = 1) 
ORDER BY node_title ASC, node_changed DESC

Also the same.

It seems that my database was messed up somehow. I now have it working on a clean setup. I'll investigate a bit more if this was triggered by some other module or some configuration settings and will let you know if that was the case.

Good news. I could reproduce the issue. The described behaviour occurs if the file aliases module is used (I used version 6.x-1.1). I'll file a bug report there (#909194: Breaks image_gallery views if activated.).