Hi,
try to check the size of the images - e.g. try to add some small images cca 200kb - to know the upload and add is working
try to check if you are able to write data to server (if there are no permissions issues, if there is still space for new images)
try to check if the thumbnail of the image is stored on the server - see the original image which should be displayed instead the "no image" and try to see, if there is a folder "thumbs" in the folder where the original image is stored and if this folder includes thumbnails on the server
Jan