Note written on January 2nd, 2015: pLoader still works with Piwigo 2.7, but its development has been stopped since July 2010. It is recommended to use the browser uploader instead, which offers equivalent features (drag & drop, chunked upload, progress bar) and does not need any installation.
pLoader is a software dedicated to prepare your photos and transfer them to your Piwigo installation:
select photos from your file browser
automatic preparation includes resizing for web display, thumbnail creation, automatic rotation
complete informations such as name, author or description
create a category (ie album)
transfer the selected photos to your remote Piwigo photo gallery
Your photos are visible on your gallery as soon as the transfer is done.
pLoader for Mac is only compatible with MacOS X 10.5 (ie Snow Leopard). If you don't want to use the web upload form, the best solution is to use either iPhotoToPiwigo or the Lightroom plugin (if you have Lightroom).