Hello,
generally speaking, JoomSEF does not modify image path, so this should not be a direct cause.
However, if you are using relative paths for the images, with enabling SEO URLs, this may stop to work.
Let me give a short example.
Lets say your path to images is images/some_image1.jpg.
This works fine as long as your Joomla path is mysite.com/index.php?....
However, one this changes e.g. to
mysite.com/my_portfolio_category
then the path images/some_image1.jpg will not be functional anymore, as the mentioned directory is "one level lower".
So, to make sure images and other resources work fine both with non-sef and sef URLs, make sure not to use relative, but absolute paths.
That means, make sure the path to images folder as mentioned above is
mysite.com/images/some_image1.jpg
(or at least /images/some_image1.jpg)