Your FeatureClass has to have a field Named Picture_1 and has to be Raster. Then you can attach 1 picture. You can also have several Picture_2, Picture_3 fields with BLOB as format, but those you can see in ArcMap, but you can then have multiple picture in Mobile attached to one object.
There is no "out of the box" function in Mobile to attach a photo directly to the object....you have to have the field above and the "browse" for it.
But it will automatic syncronize them back to ArcMap and to your geodatabase, where you can use them.