Snapable image corners for any number of tools or, more specifically, the elliptical marque tool in Photoshop. For those occassions when preciscely placing the round peg in the square hole is de rigeur, or however that is spelled.
This issue would perhaps be better-suited for a hypothetical(?) "Photoshop Upgrades to Improve Workflow" category. It relates to the idea of implementing a customizable, comprehensive right-click menu system—essentially a Drill-down Multi-level Nested Context Menu. This feature would display all possible next steps (editable by the user) when right-clicking on any tool or element.In this specific case, it would be beneficial if the designer could right-click on either ruler, and one of the options would be "Enclose entire image in guide lines" or something similar. This would streamline the process of adding guides around the image perimeter, saving time and improving workflow efficiency, not to mention introducing Ye Olde Photoshope to the 21st century. In the meantime - while awaiting that auspicious event - here are a couple of images as requested.
More complex contextual menus might be useful to some users, but for the case you describe I suspect you may be better off foregoing Selections and using Paths/Vector Masks instead.