Since there is no framework like ImageCaptureCore on the iOS platform available there are three options I think:
- As stated in this question there may be the chance of talking to the device on low level. I don't know if there are any frameworks or standards that work for cameras or taking photos with cameras.
- Depending on the camera you want to trigger there also might be the way of connecting your iPhone to the camera via cable to the trigger port of the DSLR. Specifications for the different manufacturer's specifications can be found here.
However I think you don't want just to trigger the camera but to transfer the taken images.
- You could use a SD Card with an integrated access-point to connect to and copy the image via a high level protocol.