Normal users have a limit on how many pictures they can upload.
Under common circumstances, this feature has two uses: the first is to stop spam, since the spammer can't upload more than a fixed number of pictures in a day; the other is that on the original Danbooru, users can buy with real money a permanent pass that disables this limit.
On One Yukkuri Place, picture spam rarely happens. Spammers prefer forums because forum posts don't require administrator approval and they can put more links/text in forum posts - advertisers can track you through links they own.
As for the other use, OYP doesn't have a payment system and levels (gold, platinum, etc) are given based on trust and user activity.
If you upload consistently and in high volume (hitting the limit often), the administrators will probably notice and give you Gold and limitless uploads pretty much immediately.