I'm having the exact same problem - some images are uploading and some aren't... I have also lots of files appearing on the server upload area that are called for example :
Update - I've just discovered that the upload is timing out with large files - so can i change the allowed upload time anywhere ?...
Update 2 - I've just increased my server file size limit and this works -
There is an issue with compressed jpegs - I have a jpeg which is 8 mg, but when opened in Photoshop for example it becomes about 40mb - does digital file upload uncompress on loading perhaps ?...