I looked around for an answer on transferring photos directly from a camera to the Nexus 7 and didn't find anything very definitive or easy.
I took my Panasonic camera and used the camera cable to connect to my OTG cable and connected to the Nexus 7.
My camera has transfer options of PC or pictbridge. I was pretty sure the PC option wouldn't work, only PC or Mac. I tried the pictbridge option and up on the Nexus 7 screen came the choice of Nexus Media Importer or Gallery. I tried Media Importer and that did not work. However, I then tried Gallery and that did work. I was able to import photos from my camera directly to Nexus 7. I hope this is useful for others, whether it will work with other cameras, don't know, many do have the pictbridge option now so that may be the key.