Hello fergusPearson,
Thank you for using Apple Support Communities. Sounds like you’re unable to upload some of your photos to iCloud using your Windows PC. Are you using iCloud for Windows? If so, we’d like to help out and recommend some steps to see if we can get this issue sorted out. First, make sure that we’re properly following these steps to upload photos to iCloud: Set up and use iCloud Photos on your Windows PC - Apple Support
In iCloud for Windows 10 or later
1.) Open a File Explorer window.
2.) In the Navigation pane, click iCloud Photos.
3.) In another window, open the folder where you currently store the photos that you want to add to iCloud Photos.
4.) Select the photos that you want to add.
5.) Drag them into the iCloud Photos folder.
You can view the photos and videos you uploaded in the Photos app on iCloud.com and on all your Apple devices.
You did mention that you’re unable to upload them to iCloud. Are you receiving an error message? If so, what does the message say? These details can help us better understand what is going on. That way we can recommend some steps you can try. Also, if these photos are already on your iPhone and you wish to sync them to iCloud, you can simply just enable iCloud Photos to sync them. Just in case, this resource can show you how: Sign in to iCloud on all your devices - Apple Support
We hope this helps out. Feel free to keep us posted. Thank you for using Apple Support Communities. Take it easy.