is not that FTP has problem , before you can send file to image folder , ftp needs to read files from Images folder , and you got 24k images.

maybe you want to create sub folders and upload your new images to those sub's ?

thats what i did with one zencart , due to many suppliers .i named each sub's after the supplier name and this way is easier for site owner to upload either through zencart , or through FTP ..