Are the sizes correct when you open the .idml in CS6? Have you used any frame-fitting commands or options on these images in CS6?
There seems to be a big change in behavior in frame fitting in CS6, and that might account for a change when opening .idml in a previous version. One thing you need to understand is that while previous versions may be able to open .idml from a later version, there is no guarantee or expectation that the translation from version to version will be perfect, expecially when unsupported features are involved. Depending on .idml for a collaborative workflow is asking for trouble and if you need to share files everyone should really be working inthe same version.
The size are wrong even if I open it in the same CS6.
My pictures are mostly set to 100% scale and are mostly smaller than the frame itself.
But in that IDML, different things happen.
Some pictures scale from 100% to 156,25%. At the same time in the same file some pictures keep 100% scale.
Can you show us a before and after screen capture of the same file opened in CS6, with the image selected using the Direct Select tool (white arrow) and the scale fields in the control panel showing?
The only clue I see here, and this is definitely very strange, is that in the first view I see the tab says (Converted) in the file name. That tells me the file was not originally created in CS6, and it probably has not yet been saved in CS6.
Converting legacy files, especially files from CS3 or CS4, by opening the .indd file in CS5 or later can have some unpredictable results. I recommend exporting the files to .inx or .idml from the original version and opening that in the newer version for conversion. I'm also curious to know if your results are better if you do a save as on the current file (and save it with a new name so you don't overwrite the original version) and then do your export to .idml.