Hi,
to do this you can make simple code modification.

in file /administrator/com_jsn/helpers/fields/images at line 151 replace this

$foo->image_ratio_crop = true;

with this

$foo->image_ratio_crop = false;

Now with normal upload (no crop/webcam) the parameter height/width will work as max-height/max-width, so you can set for example:
height=your fixed height
width=100000000000

now all images have fixed height and variable width.

Sorry,
try this:

replace this at line 151 (main image) and 141 (thumbs) this code

$foo->image_ratio_crop = true;

with this

$foo->image_ratio = true;

Hi again,
It works almost... but on the list 'User Lists (table)' site the thumb's width remain the same and its height is changing. I set the thumbs height and width in the 'Option Image Field' as you suggested. I tried to change the thumbs height and width in the image.php and clear the caches etc. but still the height is changing.

It works fine with the main image on the profile sites.

Hi,
now all the images have a height of 250pixel, the problem is CSS, in list template file there are these lines:
.listusers img{width:180px; margin: 20px 20px;}
.listusers .name,.listusers .formatname{font-weight:bold; margin-top: 25px;}
.listusers .formatname{font-weight:bold; font-size:16px;}

you can replace with something like these:
.listusers .avatar_mini img{width:180px; margin: 20px 20px;}
.listusers .kunstvaerk_mini img{width:auto;height:250px; margin: 20px 20px;}
.listusers .name,.listusers .formatname{font-weight:bold; margin-top: 25px;}
.listusers .formatname{font-weight:bold; font-size:16px;}