iphoto and icloud dimensions and size clarification

The version of a photo in your iCloud Photo Library should never be of lower resolution than the version in the Photos app on any of your devices. Where did you see this "reduction in size and dimensions" that you noticed?

The documentation for iCloud Photos does not mention the size limit for edited photos.. And I do not remember, that it used to be this way previously. For me, this behavior is very surprising. I am getting the same result as you observed, when downloading from wwww.icloud.com with Firefox.

If it is intentional, it should be mentioned here: https://help.apple.com/icloud/#/mm6db8290fc4https://help.apple.com/icloud/#/mm6db8290fc4

Click the Download button to download the most compatible version of the photo or video.

The most compatible version will be in a standard file format, JPEG for photos and MP4/H.264 for videos. This version will also include edits made to the photo or video since it was added to iCloud Photos.

The size limit of 2048 pixels width or height is the same as for Shared albums, and there it is documented: Use Shared Albums in Photos - Apple Support

When shared, photos taken with standard point-and-shoot cameras, SLR cameras, or iOS devices have up to 2048 pixels on the long edge. Panoramic photos can be up to 5400 pixels wide. You can share GIFs that are 100MB or smaller.

Hi thanks. So are all iCloud photos HEIF?

Not necessarily. iCloud Photos (Library) is storing the original photos in the format that they have been uploaded to iCloud.

If you are now having HEIF media in iCloud Photo Library, your iPhone may be set to record the photos and videos in the HEIF format. An iPhone with iOS 11 or iOS 12 can handle HEIF; and if the model is iPhone 7 or newer the camera can record directly in this format.

If you want to stick with more compatible media formats, set the iPhone to record the media in a more compatible format: Using HEIF or HEVC media on Apple devices - Apple Support

Your 2011 MacBook Pro cannot run macOS Mojave (10.14). According to the Mojave technical specs document https://support.apple.com/kb/SP777?locale=en_US the oldest Macs that can run Mojave are from mid-2012.

It can run macOS High Sierra (10.13.x), which can run on "MacBook Pro (Mid 2010 or newer)" according to the respective tech specs https://support.apple.com/kb/SP765?locale=en_US and can handle HEIF/HEIC/HEVC files. Whether your 2011 MBP is otherwise up to the task of running High Sierra, I couldn't say.

Also, I've done a little more testing to convince myself that iCloud Photo Library stores full-resolution images and conveys them and their edits among devices:

I took a picture on my iPhone X (back camera, 3024x4032) and let it upload to iCloud Photo Library.

On my iMac (Mojave with Download Originals enabled), it downloaded as an IMG_xxxx.HEIC file at 3024 × 4032.

On the iMac, I made some distinctive edits with Markup and tone controls, then let the edits upload.

On my MBP (also Mojave but with Optimize Mac Storage enabled and likely full enough to need noticeable optimization), it again downloaded as 3024 × 4032 with the edits intact.

I'll leave you in léonie's capable hands for troubleshooting the apparent corruption and partial transfers.

Try downloading the unmodified original from iCloud (by clicking-and-holding on the download icon and selecting unmodified original from the resulting dialog box. In my quick test, that gets me a full-resolution version.

I don't know why, but the default (or explicitly chosen "most compatible including edits") download is limited to 2048 pixels on the long side. If that fact is documented somewhere, I haven't found it yet. I also don't know how to get a full-resolution download that includes edits.

To convince yourself, that the HEIC files are of the same quality, convert a few with the online converter. https://heictojpg.com/

Could your MacBook Pro be upgraded to High Sierra or Mojave? Then you can view the HEIC files on your Mac directly in Preview.

Even iPhoto 9.6.1 will be able to work with the HEIC files, if you upgrade to High Sierra or Mojave.

Can I ask you another question is it safe to run iPhoto repair? Just wondering if that may fix some of my corrupted partially greyed out photos.

A repair can help with a library corruption, but you should always be cautious and make a backup copy of the library, before you try to repair it. If a library has a corruption, that is beyond repair, then trying to repair it can make the damage worse.

And when you repair a library, do it only on a volume with plenty of free storage. Repairing or rebuilding can temporarily increase the size considerably. The file system of the drive should be MacOS Extended (Journaled).

iPhoto has several First Aid repair/rebuild options. The first option to try is the repair of the permissions. Only if that does not help go for the repair of the database.

A Mac with El Capitan installed does not support HEIF files. To look at the heir file you downloaded, try the online converter at https://heictojpg.com/