I feel really stupid now... I already managed to position the graphics itself where I wanted it using the x/y positions, so finding the position/offset input boxes for dialog graphics wasn't the issue, the problem was that that placed all the dialogue graphics below the visible workspace, which made it "impossible" to set the dialogue area within the dialogue image, even with the Preview tab dragged all the way down. However, I completely missed the ZOOM buttons in the top right corner (I kept looking for sliders) until my eye caught them in your screenshots! :O And that was all I needed - a way to navigate to the graphics that was out of view, in order to be able to set the dialogue area within the graphics!
I'm terribly sorry to bother you with such noobish questions, but I'm afraid this is only the beginning of a long line of laughable inquiries. I assure you, however, (even though my posts may indicate the contrary) that I do my utmost to find ways around the roadblocks I hit before posting questions here. I'll only make a post when it's gone so far that big patches of skin is attached to the hair I'm pulling out. I only got one hour sleep last night, as I spent all night trying to find a way to fix this dialogue area issue, going back and forth between Glenfx's and Suro's tutorials, searching the forum and Google in its entirety, as well as having a good portion of trial and error-ing within the editor, and it all boils down to not seeing a simple button (and it's not the first time that's happened!)... I really oughta learn to go to bed already, when my brain has fallen asleep long ago..
Please bear with my senseless inattentiveness (as there's surely more to come), and one day I may reward you with an awesome traditional point&click adventure!
Thanks again, AFRLme, you're a lifesaver!