Thanks for your help. The caption is showing up now. But as the width is not specified anymore, the caption background takes the size of the text so every picture has a different caption background. I'd like to keep the width fixed, especially for the big slider on my homepage.
on slideshow.css file I can see 3 different size : medium_image, big_image and small_image.
I don't use medium anywhere. Maybe you can help me to "call" medium_image instead of big_image on those particular thumbnails. Like that I can specified a width for big_image for my slideshow and another width for medium_image for my thumbnails.