Hi philippefromsèvres,
Thanks for using the Apple Support Communities! I'm sorry to hear you are having these issues with your iCloud Photo Library. If you would like for your iMac to automatically download the full-resolution images from iCloud Photo Library, you would want to make sure that Download Originals has been selected, as noted in this article:
iCloud Photo Library automatically keeps all your photos and videos in the original, high-resolution version. Follow these steps to choose how you store your photos and videos on your device:
- On your iPhone, iPad, or iPod touch, tap Settings > iCloud > Photos or Settings > Photos & Camera, then select a storage setting.
- On your Mac, click Photos > Preferences > iCloud, then select a storage setting.
If you turn on Optimize [device] Storage, iCloud Photo Library will automatically manage the size of your library on your device, so you can make the most of your device's storage and access more photos than ever. All of your original, full-resolution photos and videos are stored in iCloud while device-size versions are kept on your device. You can download the original photos and videos over Wi-Fi or cellular when you need them.
If you turn on Download Originals, iCloud Photo Library will keep your original, full-resolution photos and videos in iCloud and on your device. Download Originals is the default setting for iOS devices with the free 5 GB storage plan and for all Mac devices.
You need an Internet connection to access an original photo or video that’s stored only in iCloud.
iCloud Photo Library FAQ - Apple Support
If you are still having issues seeing your iCloud Photo Library photos, you may also find the troubleshooting steps outlined in the following articles helpful:
Get help with your iCloud Photo Library - Apple Support
Get help using My Photo Stream - Apple Support
Regards