It is a good idea to plan from the start. The reason is because, depending on how you make your design, it may be very difficult to add a few pictures at the end.
There are a few things to consider. Mainly, if done correctly, you can deal with any image related issues at runtime. This will require carefully handling possible errors that may occur from failing to read the images and preventing those errors from crashing your program.
Basically, let's imagine you have your program. While developing your program, you access the images using standard File IO and your images are inside the project's root folder (thus no compile or runtime errors will occur while developing). When you package your program, those images will probably be inside the .jar your program is executed from. The problem is that those images are effectively in a .zip folder and cannot be accessed with standard File IO. If this executable jar is located in the same folder as your images, the program will not have an issue when you attempt to run it. If, however, the program is located somewhere other than where your images are or if your images are moved somewhere else, suddenly you will be getting IOExceptions thrown since the images cannot be found. On the flip side, if you intend on having the images inside your executing jar from the beginning, you can preempt the problems and use a Buffered Stream to the location of the executing .class. That means that when you are developing your program, no errors will occur, and when you package your program with the images, no errors will occur (unless something happens that I cannot predict). If that is the route you want to go, I can provide you with some sample code to help you out. It is actually very simple to do.
The same goes for any file reading. ALL files, whether images or text files, that will be packaged with the project must be read using Buffered Streams or the program will have a laundry list of potential issues when packaged. It is an unassuming issue that can cause unimaginable amounts of frustration since you can only debug it after packaging.