I have a schematic diagram that I created and have used in a published peer-reviewed conference paper. It is not part of the method or the results for the paper, just an explanatory diagram giving context, explaining teh terminology used in the text. I want to use the same diagram in a journal paper on a similar subject. Do I need to 1) reference the original conference paper in the article caption? 2) seek copyright permission from the conference organisers?
You should check with the original publication and the submission journal. Technically you are re-using copyrighted material so it seems little to no trouble to ask and get the correct answer rather than go ahead and correct a later misunderstanding. The journal, for instance, might care to know that they are publishing material copyrighted elsewhere.
It depends on whether you assigned copyright to the conference organizers (by signing or agreeing to such at submission time).
If you assigned copyright then the copyright lies with them and you probably need their permission (fair use provisions may apply in some jurisdictions).
If you did not assign copyright then it is still yours, and you can do what you like with it.