You can import any image by using a DSi jpeg signing tool. if you put it with your DCIM folder on you SD card, then you will be able to see them on your SD card. The import feature can only import photos from the system memory for whatever reason so you need to copy them to the system memory first. I will warn you because the system memory on the DSi is very weak and compared to today's flash memory, can handle very little writes before failing so I would recommend copying then importing the photos in an emulator and then after moving the .ppm file to the DSi SD card. If you use an emulator to make part of your Flipnote, you won't be able to upload it here without breaking their TOS (section 4). I'm not sure if custom jpegs are allowed to be imported.
You can import any image by using a DSi jpeg signing tool. if you put it with your DCIM folder on you SD card, then you will be able to see them on your SD card. The import feature can only import photos from the system memory for whatever reason so you need to copy them to the system memory first. I will warn you because the system memory on the DSi is very weak and compared to today's flash memory, can handle very little writes before failing so I would recommend copying then importing the photos in an emulator and then after moving the .ppm file to the DSi SD card. If you use an emulator to make part of your Flipnote, you won't be able to upload it here without breaking their TOS (section 4). I'm not sure if custom jpegs are allowed to be imported.