Can I use images from my published papers in my thesis without copyright infringment? I have to fill in copyright form for my PHD thesis submission.
I have used some of the images already published in my IEEE/IET journal and conference papers.
I signed copyright forms during the publication process of those papers.

Do these figures fall under the copyright act?

Can I confidently say that 


  My thesis does not contain ANY material of which Copyright belongs to third parties.
You will have to read the copyright transfer form that YOU have signed. Sometimes, there are several options on whether you keep the right to reuse your work.
In my personal experience, you often have some limitations on how you can reuse your material. Sometimes, you will have to place a reference on where this paper has been published first.

However, this entirely depends on what you have signed.