I’m using gwenview to preview and manipulate images, and luckily there are further tools to add batch processing of images, especially resizing those large images before uploading them to some galleries (saving upstream bandwidth which would be wasted when resizing images on the server).
# apt-get install gwenview kipi-plugins imagemagick
Within gwenview go to “Modules” – “Batch Processing”. Click “Properties” to set the max output pixel (like 1280px), then add all wanted input images, select an output destination and start resizing your images.
If you are seeking for an online method in your wordpress galleries, try NextGen Gallery and NextGen ZCropper