have you tried manually clearing out the system temp folder? Windows does a rather poor job of managing the orphaned files in there and over time they can almost bring the operating system to it's knees. If old junk is still there, then it is possible that images from another mail will be read in because they are already there. especially if the file name is one of those Image1 type names.

Note, when an image is in an email it simply does not exist as a file until you select the email to view, likewise an attachment does not exist until you choose to save or open it. At that time the files is created in the system temp folder and included in what is displayed or offered up for the save process. Remote image have to be downloaded on access and will usually be in the Thunderbird cache.