Need help for saving to PNG files

[SharART]

I can't seem to save the extracted images that I create from jpeg files to the png format...I need this feature to save/create new layers for my artwork that do not have any background whatsoever including white...can't seem to find the right settings...any help will be greatly appreciated!
In the options dialogue in the save menu - the TRANSPARENT box is greyed out so not an option - can this feature be accessed?