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What is the file format of the photos you download to your iPhone from your PC? Try to stick with compatible image file formats - JPEG, some RAW formats, TIFF, PNG.

Occasionally photos cannot be opened, if an unsupported color sync profile is used. It is suspicious, that only photos you took with the iPhone camera can be opened. The upgrade to iOS13 and Catalina made some older codecs incompatible. Some photos and videos, that have been compatible with iOS 12 and macOS 10.14 Mojave are no longer compatible with iOS 13 and macOS 101.5 Catalina. Compare the formats of your image files to this list: About incompatible media in iMovie for macOS - Apple Support

The list has been written for iMovie, but it can be a guide for Photos too.

In addition to Mike's question about the "Optimize" feature - how much free storage is on your iPhone? You may also get this message, if there is no storage left to download the photos from iCloud. Did you upload more photos to iCloud then will fit onto the iPhone, even "Optimized"?

In Settings > Photos, which option are you using for photo storage?

If if you are using Optimise, it may take a while to download the images and all their data to be able to edit them.

It may take a week or three to fully download your library to your device. (My library of over 65,000 photos and 600+ videos is around 95gb and takes about 3 weeks to fully download)

Further to my comment, it seems that there is a difference between jpeg and jpg. But I don't understand why the Photos App doesn't recognise the jpg file?

I've uploaded other images to iCloud via PC as a jpg file then the Photo App won't open the file or let me edit the file.

However, if I've upload the same image as a "jpeg" file then the Photo App will allow it to open and edit.

I don't have a Mac but a Windows PC

That seems to be the solution.

Are the files that you "Save for Web" from Photoshop also JPGs? Or GIFs, PNG, or something similar?

I found quite a few files from Photoshop, that would not sync with iCloud, but they looked like normal JPEGs or TIFFs. But Photoshop had embedded a different color sync profile, that Photos could not handle.

I've had the same problem as you Charles.

I've been in contact with Tech Support and I shared my PC, Devices and iCloud app to investigate. Given my detailed report it seems like it's a bug with mismatched file type associations. It's being investigated by the engineers and probably will be a patch for it in the near future.

I tested a bit on my Mac. When I import a file with the filename extension ".jpg" to Photos on the Mac, Photos will change the filename. When I export the file again from Photos, the exported filename will be all capitals and the filename extension will be ".jpeg". So Photos for mac is now uploading all JPGs with the filename extension ".jpeg" to icloud. But the web interface at www.icloud.com is still working differently and not converting the extension.

On my Windows PC, when importing a file with the filename extension ".jpg" or ".jpeg" from Photos iCloud on my PC, Photos file name stays the same. It doesn't change.

When I export the file again to Photos iCloud app, the exported filename stays exactly the same.

I load a ".jpg" photo from my Canon camera into my PC DropBox then I go into my iPhone, go to DropBox and save the photo from DropBox into Photos and then I go to view my photo and Photos says "Cannot load Photo".

So I don't know what to think.

However Leonie, I do think the interface www.icloud.com is working differently and so is Photos on devices.

I can however, use the Snapseed Photo App on my iPhone and import that photo into that App, the one that Photo won't load and Snapseed has a message that - "This is image is a read only. Saving will create a copy." I make some adjustment and Snapseed creates a copy which Photos then will open.

It's really strange.

It's nothing to do with file format compatibility. The photos I've uploaded to iCloud via desktop are all jpeg files, which were originally taken with my iPhone in the first place.

I've had this trouble since iOS 13 upgrade not before.