I've managed to reach it merely by uploading two JPEG banners and avatars for two communities: ^1 ^2
{"data":{"error":"rate_limit_error"},"state":"success"}
Considering that the image upload previewer isn't 1:1 with what the website sees, and images cannot be repositioned after upload, this matters.
That seems quite trivial to meet, so I'd say that it should probably be specific to the kind of upload. Nobody should be posting more than 6 image posts per hour, but submitting images to a community's decorations is different.