Nagif,
Hmm, it seems like you have Optimize iPhone Storage turned on. This is where a low-res version is stored on your phone, but the high-res version is on iCloud.
To check go to Settings > your-name > iCloud > Photos. Is Optimize iPhone Storage checked? Is so you have 2 options:
1) Turn off Optimize iPhone Storage. This may require substantially more storage on your phone.
2) Click the photo to download the high-res version before posting elsewhere.
To check your local storage:
How to check the storage on your iPhone, iPad, and iPod touch
Details on photos in general:
https://support.apple.com/en-ca/explore/taking-managing-photos
Good luck.