What i have found so far is:
If the Image control gets the Data from a text field, the pictures show up fine.
If the Data is a calculated field (in this case with CONCAT( [folder].[folder], [main].[filename] ), the pictures get shown as grayscale.
LO 3.5 Base and before showed the pictures as they were supposed to be in both cases.
What setting needs to be changed on my side to regain the behaviour of the image-control from the older LO versions?
my system is win 7 64bit, LO 184.108.40.206, Java 1.7 32bit, embedded HSQLDB