Enable ACP option to specify image compression level

phpBB uses the Plupload module to handle uploading of image attachments when the maximum image dimensions are set to a value other than 0px x 0px. Plupload performs client-side image resizing. Once the max image dimensions are greater than 0px by 0px, Plupload also performs compression on all images, even if the image is larger resolution than the max image dimensions configured by the board administrator (and thus Plupload does not perform any client-side image resizing.

Plupload has a quality setting for image compression, expressed as a percentage. A higher quality setting will result in better image quality but larger file size.

The default quality setting currently used by phpBB is 85. The Plupload default is 90.
My testing suggests that the Plupload default of 90 produces approximately the same image size as the original source image without observable image degradation to my eye (and likely the eye of most board administrators).

A quality of higher than 90 generally results in a LARGER file size than the source file, assuming the image dimensions are equal.

Board administrators might wish to test different quality settings according to their needs, eg they want to reduce space taken by image attachments or want to preserve higher image quality.

I propose:

• a new ACP option to allow configuration of the image quality setting for Plupload
• the default quality should be set to 90% to accord with Plupload's own recommendation.