I started with SIG free. It didn't work. Than I bought SIG PRO. Still the same.
Thumbnails are blanks. When I click on it, it's opening fine. I checked the name of the pictures. Even create one with the name test. Same thing. I did change my cache folder to 777. Still nothing. My pictures extension are jpg. No special characters no caps etc. No difference.
Can anybody help me with it please?!? :( :( :(

here is the web adress
www.dancestudiojump.com/index.php?option=com_content&view=article&id=109:galery-test&catid=36

You have a CSS issue. Your template CSS has a statement ul li img {background:none !important;} Since SIG Pro loads the thumbnails as background images, this statement overrides the plugin's CSS and you have the problem mentioned.
You will have to find a way to overcome this CSS statement in order for your thumbs to show.

Ha! So easy to say:"You will have to find a way to overcome this CSS statement in order for your thumbs to show" - but how to do it? This is a question!!!!
I'm a new to that kind of process  ??? ??? ???

Please provide us with a link to your gallery page.

I had these exact symptoms..... After trying some things in the previous posts, it ended up being that my file names had spaces in them.

Hello, I am French and was understand everything.
I have the same problem thumbnails do not display
Here is the link towards my gallery

bonjour,
je suis français et ne comprend pas tout.
j'ai le même problème les vignettes ne s'affichent pas.
voici le lien vers ma galerie

www.sativa-rock.fr/index.php/galerie-photo

Check that your cache is writable and make sure that in the names of the photos you don't use special characters like caps, slashes, etc.

Thanks it's works