How to display always the same thumbnail for a category in c

[siyman]

Finally solved, in this version you have to change the file:
Siyman and Jan, thanks a lot for you disposition to help me!!

Well, in fact I have to thank you. You saved me a lot of time because I didn't upgrade the gallery for over a year, was sure I have to find another solution for newer versions and you are serving a nice little hint. Thank you, aaronrrc!