Wow I wasn't even aware there is a photo limit, but that's probably because I have only been playing for 2 months. I'm pretty sure it will be full soon though XD
I've reached it a couple times and I'm probably nearing a third. The limit is around 3,000 photos--even if the SD card still has TONS of space left, you have to remove the pics before you can take more. It's such a pain.
I should probably upload my pictures from my 3DS to my computer again and then delete most of them. I remember taking a lot during Festivale and Bunny Day.
i reached the limit about 2 months in, and it's just so hard transferring each photo 1 by 1 via twitter that i just gave up lol but now that i have a new comp, i think i can finally try the sd-card slot thing, even if i have no idea what im doing D: