Most modern cameras produce photos of 7 or more megapixels.
The filesizes can be up to 4 MB for a 7 MP picture and much more for larger pixelsizes.
As soon as a visitor watches one of your pages with a photo on it, the photo must be downloaded from the server. This takes time.
A photo of, say 7 MegaPixels, i.e. 4 MegaByte, will be displayed on the screen no larger than the Full Size you specified on the Photo Albums -> Settings admin page. This will in most cases be no larger than 1200 pixels wide by 900 pixels high. That is approx. 1 MegaPixels, that converts to about 500 kilobytes.
If you keep the full 7 or more MB photos on the server, it not only takes up much diskspace but also takes a long time to download. The pictures will be downsized on the visitors computer to the display size.
On the other hand, if you kept pre-downsized pictures on the server, the download time will be much less (in the order of 10 to 40 times!), and it also takes up less diskspace.
Therefor it is strongly recommended that you save the photos on the server in the largest display size you will use on the site, and not bigger than that. Also not smaller because stretching a picture, strongly affect its display quality in a negative sense.
You can make WPPA+ automaticly downsize the pictures you upload or import to the specified Full Size. That sounds nice, BUT…
You may get Out of Memory errors from the server. I will explain this:
Although you can upload files of up to 10 MB without problems – and photo files also – there is something else to be aware of.
Whether or not you select the Resize on Upload checkbox, the server has to make a thumbnail image, to be used for displaying the cover or the contents of an album.
To make the thumbnail, (or the resized full image), the server needs to convert the jpg, gif or png file to a true color bitmap representation in the servers memory. Full color means 3 bytes (one for each color: Red, Green, Blue) per pixel. So an input picture file of 7 megapixels (although maybe 3.5 megabyte in filesize) will require 7 * 3 = 21 megabyte of temporary internal server memory, or even 33 MB for an 11 MP picture.
Standard WP configurations specify the use of maximum 32 MB server memory. You can increase that to 64 MB by changing the wp-config.php file. This will be enough in most cases. BUT…
Many hosting providers do not allow you to use more than a certain amount of memory. If you try it, the program stops with an Out of Memory error message, and leaves you with in inconsistency in your WPPA+ system.
The latter is no problem, just enter the Photo Albums -> Upload Photos or Import Photos admin page and any inconsistencies will automaticly be fixed. There ar three things: Databse entry, Full size image and Thumbnail image. If one or two of these are missing, the others(s) are deleted, you have to upload the corresponding photos again. When importing, the original image will not be deleted in case of a failure even when Delete after import is checked.
Conclusion: Use Resize on Upload to be sure that photos are saved on the server in the most ideal way, but do not expect that it is a monkey proof way to add photos to your WPPA+ installation.
Find out what your maximum available server memory is, or, even better, what the maximum megapixel size is that you can upload or import, and downsize your photos before uploading. There are many good performing free programs available to do this for you.