I'm not sure I understand what you want achieve...
DokuWiki uses either libGD or imagemagick (version 6 I think) to resize images, and, as far as I know, only to resize images. See
https://www.dokuwiki.org/config:im_convert for the pros ans cons. DW resizes images when it meets the {{image?XXxYY}} syntax, and when it generates previews in media manager. From within DokuWiki you cannot use it for anything else. There is no way to automagically convert png to jpeg for example, unless you code your own plugin.
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Does anyone know this software well and can vouch for it's image quality and stability?
1. It can resize overly large images uploaded to Opencpn dokuwiki.
2. It can change them to jpeg.
3. It can compress them to the value we want.
Imagemagick is a well known program. It's the swiss army knife of image manipulation.
If you run imagemagick from outside DokuWiki (ie command line on your server), then :
1 yes
2 yes
3 I think so
for example, on a unixish server, you could launch that sort of command (untested) :
find data/media '(' -name '*.png' -o -name '*.jpg' ')' -a ! -name '*-30pc*' |
while read name ; do convert "$name" -resize 30% -quality 80 "${name%.*}-30pc.jpg" ; done
From any foo.png or foo.jpg file it creates a new jpeg image named foo-30pc.jpg which
is 30% of the original size (in pixel, not bytes). This is just a starter code, you'd still have
to adjust the ownership of the created image files (and eventually selinux context). By itself,
this would have no effect on your pages, they would still use the original images. You'd then
have to edit all pages so they use the *-30pc.jpg image; maybe with the help of the batchedit
plugin... See
https://www.dokuwiki.org/plugin:batchedit .
Both may be possible if you write your own plugin. Definitely impossible otherwise because
DokuWiki does not do that sort of things out of the box, and I don't think there exist a plugin
that does that.