Experiencing a similar issue, intermittently
THE BEST WORKAROUND/FIX appears to be to close iPhoto and delete ~/Library/Preferences/com.apple.iPhoto.plist
It will effectively reset iPhoto, although you'll keep your existing events, and it may cause iPhoto to 'see' your iPhone again. It appears to have worked for me; I left iPhoto open for a bit and it finally displayed my iPhone and let me import photos
I discovered this via this thread on the Apple iPhoto support forum
As per "Kio Dane" , seeing if Image Capture 'sees' your iPhone and will pull photos from it is a great first step.
As part of my troubleshooting, I set iPhoto as the default application to open when iPhone is connected (via Image Capture preferences)
When I plug in my iPhone, iPhoto opens, but will not display the iPhone in the sidebar to display & import photos
The reason I was using iPhoto in the first place was because I like to keep ALL my iPhone photos on the iPhone, but import new ones to my machine whenever I sync. With Image Capture, which is more 'stable' (almost always 'sees' the iPhone), it doesn't detect already imported photos, which can result in duplicates.
Picasa will detect duplicates, but is very slow to scan the iPhone each time it is connected to sync.
It appears that the SOURCE of the issue around iPhoto not seeing the iPhone has to do with non camera photos being stored within the camera roll (ie. screen grabs, or images saved from Mobile Safari) - some people have reported success in getting iPhoto to see their phone again by deleting any non-camera photos from the camera roll. I hope Apple comes up with a proper fix for this.
When you open up the folder containing the photos in Windows Explorer, click the down arrow next to the button I circled on the top right of the window, and choose "Extra Large Icons":
This will zoom in your photos to a much larger size.
If you want to quickly evaluate a photo without opening a new program, you can also click the button to the right of the previous one mentioned (circled here):
This will enable the preview pane to be visible on the right side of the Explorer window. When you select an image, it will load a higher-resolution preview of the photo on the right side. You can resize the preview pane to get a better view of the photo, as needed.
It is worth noting, based on your previous comments, that this solution will display the images in as high of a resolution as the zoomed size will permit. This includes the preview pane. Conceptually, the preview pane, if resized to be large enough, will display the photo in full resolution.
Best Answer
You actually don't need to download anything extra at all. Apple includes an application called Image Capture as part of the default OS X install. You can use it to import pictures from your camera/iPhone, as well as scanning in photos from a scanner.