Unfortunately this is not possible natively so you need some custom modification:
- Set to "Normal file input" the parameter "Upload Method", now you will see a simple upload field
- With simple upload field the image will be automatically cropped. To prevent this you need to make some changes to our code, in file /administrator/components/com_jsn/helpers/fields/image.php replace at line 182 (main image) and 192 (thumbs) this code
$foo->image_ratio_crop = true;
with this:
$foo->image_ratio = true;
Now with normal upload the parameter height/width will work as max-height/max-width