Are you synchronising data via iCloud with other devices - such as an iPhone, Mac computer or another iPad - or accessing Photos using another computer? If so, disabling iCloud Photos from iPad settings will impact access to photos from iCloud.
If you have disabled iCloud Photos on your iPad, any photos already uploaded will remain accessible in iCloud.
You may find this support page will help with understanding how to set-up and use iCloud Photos with your iPad:
Set up and use iCloud Photos - Apple Support
The iPad User Guide is also a rich source of information:
Use iCloud Photos on iPad - Apple Support
Working from memory (as I have not configured my own devices to only keep optimised images) - while iCloud Photos remains enabled in iPad settings, images that are only locally saved in optimised form on your iPad should be able to easily downloaded at full resolution to you iPad from iCloud:
Settings > Photos > Download & Keep Originals