Students need to be aware of copyright issues if re-using their own previously published material within their theses. If you have published your research, for example in a journal article, you will have come to some sort of copyright arrangement (even if you are not aware of this) with the publisher of that article. You would need to seek appropriate permission for re-using significant portions of material if your thesis is to be published online / placed in an online repository.
If you use small amounts of material (text, images or otherwise) you may likely acknowledge these as you would any other source that you would reference without seeking specific permission. However, what would be deemed a 'small amount' or substantial part can vary depending on a number of factors and you should seek clarification if there is any doubt. Decriptors such as 'small amount', significant portion' or 'substantial part' are not defined legal terms and would be open to interpretation by examining committees, publishers or even courts of law.
If you are not able to get permission from a publisher or other copyright holder, you will need to consider other options than publishing your full thesis online. Many publishers accept that theses are published online and will grant permission. However, permission must be sought. The Library, via Enlighten: Theses, provides some information about including third party copyright material in your thesis and about restricting access to your thesis.
There are differing views as to whether it is appropriate (beyond the issue of copyright) to include entire pieces of work, such as journal articles, within your thesis as a chapter or chapters. You are strongly encouraged to discuss this with your supervisor and to seek advice from your Graduate School if this is relevant to you. In some subject areas it may be encouraged while in others there is the view that including one or several pieces of work in their entirety does not contribute to the sustained and logical argument that is required of a thesis. This is not specifically prohibited in the University regulations - rather, it would be the call of the examination panel as to whether as to whether this was appropriate.