The image uploader on the Classic Desktop Site uses client-side scripting to navigate your computer and select the images to upload. There seems to be a limitation in this technology with images at very high resolutions, such as over 5,000 pixels wide.
Images this large are only really useful when viewing them on a 4K TV or printing them at poster size.
If this issue is affecting you, either:
- Resize your images down to something more appropriate, with a maximum size of 2048 pixels.
- Use the native browser uploader - with no client-side scripting (or useful navigation features) - here: