I am using RSD V2.5 Build 2361. Build 2357 did not have the update to the img "src" element working yet.
2357 is the latest one available for download from CC
Quote from BOB
We pushed new versions of Responsive Site Designer, Bootstrap Builder and Foundation Framer live today with a different approach for the picture element. Following our research, we decided to follow the specified implementation
In short we concluded that:
1. that both issues cannot be fixed at the same time,
2. considering the increased browser support of the responsive images, the best solution looks to use src.
3. the double downloand 'might' only affects Opera and IE11 and older browsers — 'might because depending when picturefill kicks in, the double download is cancelled.
Go ahead, get the latest of the best and let us know how it goes!