XF 1.5 Image resizing

Hello, in ACP > Options > Attachments I have file size set to 10MB and dimensions set to 1600 x 1600. From what I've read I understand that all images under 10MB uploaded should auto-size to 1600 x 1600 but this isn't happening and users are getting this error:


Is there something I'm missing? I contacted Nimbus and they said to raise the matter with Xenforo support.