Thumbnails only generated after first view

Hello everyone,
I use the Gallery to manage the pictures for a website. That means that the picture-calls come from another site (using MediaBlock to fetch them).
The problem is, when I add a new picture, and then call it from the extern website, the thumbnails aren't generated. The result is that I see the original picture size on the extern website.

However, when I add a picture and click on it to watch it at the Gallery, the thumbnails are created. After that, when I call it from the extern website it works.
Also, when I go into "Site Admin -> Maintenance -> Build all thumbnails/resizes", the extern website call works too.

That means that the thumbnails aren't generated directly after import. Should it be like that? Is there a way to "force generate" them?

While adding the picture, the checkbox "Create thumbnails now" is checked. I tried to change toolkit: Imagemagick. Doesn't work either. Uninstalled GD and Imagemagick and reinstalled only GD. No change.

Thank you,

Kakmael

Gallery version = 2.3.1 core 1.3.0.1
API = Core 7.54, Module 3.9, Theme 2.6, Embed 1.5
PHP version = 5.3.3-7+squeeze3 apache2handler
Webserver = Apache/2.2.16 (Debian)
Database = mysqli 5.1.49-3, lock.system=flock
Toolkits = Gd
Acceleration = none, none
Operating system = Linux AB-SRV05 2.6.32-5-amd64 #1 SMP Tue Jun 14 09:42:28 UTC 2011 x86_64
Browser = Mozilla/5.0 (Windows NT 6.1) AppleWebKit/536.5 (KHTML, like Gecko) Chrome/19.0.1084.52 Safari/536.5

Hello,

And once again, this works You are great suprsidr!

Also changed GD to imagemagick.

Thanks a lot,
Kakmael