I said that premise should be the first thing you address, but someone's great comment made me think I should revise the part about it being first. Purpose and premise, while extremely useful in the search for good design, don't need to define your process. Especially if you have a method that already works for you. For some people the exploration and discovery process is something that comes naturally. Great ideas can come from many sources and sometimes in surprising ways.
That said, at some point along the road, you should be able to answer these questions or you'll have a hard time pushing your paintings or designs to the next level. Go ahead and look at premise first if you don't know where else to start, but if you have a feel for where you want to start, do that first and then try to address premise and purpose retroactively.