Well, for this to be so you'd do well to have a grid. A snap-to-grid function as well.
It'd be very nice to have a lot of cool features for tile management.
For instance, setting the grid, you can view the grid, make it inivisible, snap to or not snap to grid, etc. It would also be nice if you had a "view tiled" window. In other words, it show the selected region or the entire image, or just the area on the present layer, tiled (repeated next to each other) to allow you to, as you design a tile or tiles be able to easily work with it and edit it to be a good repeated graphic which is good for background art or something else (such as a video game tile).
It's always not good to have a tile that you can tell easily where it begins and ends, and instead of manually copy and pasting it next to itself over and over to see how it would look when tiled, a preview window to "View Region Tiled" would help you when putting together such graphics/pixel-pushing. Maybe make it able to click and move tiles around, as well, without having to select, copy, paste, move, delete, etc. areas defined by a grid (a height and width, usually 16x16).