When an image is pasted into the Confluence (4.x) editor it is attached in the format:

imageYYYY-MM-dd hh:mm:ss.png

When using Windows, it is impossible to edit this image and upload it as a new version. Because Windows does not support a colon ':' in a file name you cannot save the file locally with the colons. Thus, the file name will not be in the required format to make a new attachment version.

The only workaround is to edit the properties of the attachment and remove the colons.

It would be better to avoid this problem by removing colons from the pasted images. Perhaps with the format:

imageYYYY-MM-dd_hhmmss.png