Did you try the latest DAVdroid version?
A few days ago I switched from CalDAV-Sync and CardDAV-Sync to DAVdroid. Earlier today I created a contact on the phone and added a picture. When the contact syncs to my ownCloud server, the picture disappears. This did not happen with CardDAV-Sync.
The other direction works fine. I can create a contact on ownCloud, add a picture, and it syncs to the phone. I can also create a contact in KDE’s KAddressBook, add a picture, and it syncs to the phone.
I see there are some similar issues here about degrading images. I’ve opened a new issue because, in my case, the image is deleted.
Android: 4.4.4 (on Samsung Galaxy Note 4)
Seems to be an OwnCloud problem. Please follow up at https://github.com/owncloud/contacts/issues/1011.