As frosticek said, there is no easy solution, beside to create an image manager to upload and manage category images i can add that, even making a plugin to do that, you still have to edit a lot of lines in theme files to show the images for categories.
What i mean is, if you don't have coding skills is better to forget about it (editing theme files like funtion.php can damage your theme if you don't know what are you doing) and look for a theme that have this option.
If you do have coding skills, with some time, you can do it yourself.
About the sentence "(that osclass shoulded have by default)", even if osclass include the functions, will be only available with bender theme, if you change theme you can't use the functions.
