Why will my image not upload to the media library?

Check these:

• File size: images can be up to 5 MB each (videos, documents and 3D files up to 100 MB). Large phone photos often go over this; resize or compress them first.
• File type: the upload window lists the allowed types. Use a JPG, PNG or another listed format.
• Storage: go to Settings → Web Settings → Media Settings and look at Capacity and Storage used. When your storage is full, uploads stop. Delete files you no longer use or upgrade your plan.

The upload window also shows a warning when you are close to your storage limit.