One of the the things I have found incredibly frustrating about how Apple's iCloud works is what it does to the file size (and therefore the pixel count) of images that it moves around. As far as I can tell, Apple tries (quite successfully, I might add) to manage the space one has on one's devices by downsizing the image when it feels it needs to.
I discovered the hard way that the pixel count of an image that was taken by my wife's iPhone and shared via iCloud to my iPhone (and to my Mac) was NOT at the original size. In addition, it mucked with the EXIF data.
I found that the only way to guarantee that I had the original file size and original EXIF data was to transfer the image directly from the iDevice that took the image to my Mac. As suggested by someone in this thread, I purchased PhotoSync, which allows transfer via WiFi, and this was less cumbersome than using the cable and iTunes.
Through trial and error I also discovered that if someone has sent me an image via an iOS Message, the best way to get it off of the iPhone is to share it via email (tap once on the image in the message and then share), because this preserves EXIF data (though the pixel count is not as large as on the iDevice on which the original image was taken).