Unable to location image field tool option when creating a PDF form

Hi all, sorry if this has been asked before but I am having difficulty with creating a PDF form for a client. There are multiple signature boxes in the form and the client wants to be able to insert a signature which is saved as a png image, however I can't seem to see the image field as an option in my toolbar. It seems to be missing. Anyone else come across this? Any help or suggestions are greatly appreciated!