Can't delete Categories, options, characteristics.

Hi,
I have just upgraded to the latest version : 1.5.4 (Essencial)
when I add a product in the back end and add it to a Category or add an option...(or anything in that right column) the delete.png is not showing (although i can see it if i go to it directly) and there is nothing that can be clicked in the "Delete" column. using the developer tool I applied this on the <img> tag:
element.style {
width: 20px;
height: 20px;
}
the image still didn't show up but i got a clickable area.
same behaviour with google chrome and safari.

Thank you for your reply Nicola.
the image is not the problem, because as I said, I can see it when I point directly to it (as you said: http://mydomain/media/com_hikashop/images/delete.png)
the problem is that:
1- the image is not showing up in the table under the Delete column where it should be.
2- and MORE IMPORTANTLY I cannot delete the items listed (that's for all in the right column of the admin product page: Categories, options...). there is nothing to click on in the Delete cell.

just sent you a PM.
thanks

ok, weird I went back into it now and the delete icon is now visible and it all works correctly.
have you already done something?

I don't understand what happened. sorry about that, it's all working now...