You can do it in paint tho it's kind of a pain, but it's easier to do in photoshop. You just splice together different cropped screenshots of the area you want a whole picture of.
The best way to do it is to have to take the least amount of pictures to splice, and to be able to zoom into the picture (I usually do 300% in photoshop) in order to see where the images should mesh. So I'll zoom in with one image about where it should be then I will hit the button to make that layer invisible, then visible, and see if there are changes where the other layer is when the new one is visible, then move the image accordingly.
In paint you can zoom, but you can't turn pasted images on and off, so just zoom in a ton and then try to line up the pixels where they look like they go. This is easiest if you crop with the intent to include "markers" in the cropped picture next to the edge. IE if you are trying to paste in a big area of grass it is very hard to line up, but if you include the corner of a building then it will be much easier.
For an 18x18
To take the least amount of pictures I usually try to start with the NW corner. The width of the UO screen will fit an 18x18, but the length won't. So when you take a screenshot of your game window eyeball the lower left corner as you stand behind an 18x18 and try to get it where the house sign hits the side of the screen. Now you need only 1 or 2 more pictures to merge with this one.
Crop out that picture and take another one - if you want the outside of the plot you may have to take one standing on the west side of the house, far down enough on the screen that the south east corner would be visible if the game window was long enough, and then another one on the east. If you can stand on the first floor of the house but still show the outside of it (ie balcony or whatever) then that is ideal. So you can take a pic of an 18x18 outside with only two crops this way.
You can open more than one instance of paint if you need, this is much easier than trying to do everything in one. Paste your new images into new paint windows then copy the areas you want into the primary paint window you're using. Have this one opened big and zoomed in so it's easier to line things up. Don't forget you can "undo".
