winkm,
One more guess.
Is Thumbnail Creation turned ON?
AdminCP > vBulletin Options > Message Attachment Options > Thumbnail Creation
I have mine set to 'Yes, with border and dimensions' (which just may be the default, I don't know).
Also, in Attachment Manager gif, jpg, jpeg Thumb column all read Yes (Yes?).
~OZ

OZ, I changed your account so you should be able to post in the Test forum, would appreciate if you could check!

thanks a lot again!

winkm,
I posted in your Test forum.
Do you have Thumbnail Creation turned ON? It should have created a thumbnail when I posted; or, at least, that's how it 'happens' when anyone posts on the OMAC forum.
Hmmmmm.
~OZ
P.S. Click on my signature; have a look at OMAC; click on New Posts, or, say the Critique forum to see the thumbnails.

Is there a solution for this (full size images in threads with thumbnail enabled) ?
thanks!

I think you can accomplish what you want by editing the postbit_attachmentthumbnail template

This is a dead, unsupported hack that hasn't been touched by JasonMerchant since 21 SEP 2007. It's meant to display the thumbnail in posts that have Thumbnail Creation turned ON, period. This hack does NOT display full sized images, just only thumbnails.
See my post #121 above concerning how to turn on Thumbnail Creation. Note that if you had Thumbnail Creation turned off and /or change the setting you will need to go to Maintenance -> Update Counters -> Rebuild Attachment Thumbnails after changing this setting and rebuild your thumbnails.

Neverbored: what should I change in this template ?? (complicated code!)

Copy and paste the postbit_attachmentimage template contents into the postbit_attachmentthumbnail template (delete the current contents in postbit_attachmentthumbnail)

Make sure you have thumbnail creation on. This will display the thumbnails for use with this hack, and in the thread will display the fullsize image (what you would normally see if you clicked on the thumbnail of an attached image), which is what I understood you were asking to accomplish...

It's a quick fix and shouldn't pose any problems other than the fact potentially a lot of thumbnails are created that aren't used if you have a lot of image uploads.

can we enable it only for some of the forums/categories?

You have to enable it for each forum you want to use it on (so to your question, yes)