Undesired behaviour of image control in Base (LO3.6.3.2) [closed]

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, Java 1.7 32bit, embedded HSQLDB

The Solution is apparently very easy :).

open the properties of the image-control.

on the first tab set the read-only property to yes.

Save the form and then the db. That’s all there is to it.