I was going to try and explain - but the help is pretty good on this - please see the following 2 soapui and readyapi! links.
ah - I just re-read your message - are you saying you get a .png file in a SOAP response and you need to pass this onto a subsequent SOAP request? Anyway - have a look at the links to familiarise yourself - if this is the case just get things setup to send a file first and then worry about passing it from a response to a request afterwards
if this helped answer the post, could you please mark it as 'solved'? Also if you consider whether the title of your post is relevant? Perhaps if the post is solved, it might make sense to update the Subject header field of the post to something more descriptive? This will help people when searching for problems. Ta