ISSUE SUMMARY:
Now upgraded to jomsocial 4.0.2 the size, dimensions and style of pictures - exept event covers - in the activity stream and member avatars (module position 'tab: Mitglieder') are not shown correcty. It looks like a css problem, but I have no right idea where to start and how to solve or override the problem as there are more than 4000 code lines in jomsocial style.css. Any assistance or solutions are very appeciated?

You find jomsocial 4.0.2 running with the above explaned problem on path domain name + /gemeinschaft

Found the solution by myself. Fixed the problem by editing the css of the gavick template gk_creativity:

/joomla/corporate173/eo/templates/gk_creativity/css/joomla.css

OLD:
section img {
height: auto!important;
max-width: 100%!important
}

NEW:
section img {
height: auto;
max-width: auto
}

/templates/gk_creativity/html/com_community/assets/css(style.css

OLD:
.joms-media--image img {
max-width: 100%; }

NEW:
.joms-media--image img {
width: 100%; }