Attribution is critical as a sound scholarly practice. Images of art or architecture should always be identified as to the artist, the owner, and the photographer where this information is known. Textual quotations should always be attributed to the author.
Many of the licenses that allow use of materials without seeking further permission, including Creative Commons’ licenses, require that attribution be made to the original source.
Acknowledgment of such licensors can be made at the end of an individual lecture. It need not be included on the particular slide or at the time when the work is used, if that would harm the flow of the instruction.