Please select "Base", "Small" , "thumbnail" images from back-end
Clear the cache and Index Managements
Or
Check the file access rights for the image folder. Files there should have 664, Folder should be on 775
Check if the image gets written in image cache media/products/cache/...
Or
This can be caused by wrong permissions set to uploaded files. Open lib/Varien/File/Uploader.php and make sure following permission are set:
Around line 219:
chmod($destinationFile, 0666);
Around line 541:
if (!(@is_dir($destinationFolder) || @mkdir($destinationFolder, 0777, true))) {
Note: This issue will also be fixed by applying patch SUPEE-7405 v1.1