Those with Linux - ensure that Kim is installed. Open directory of images, press Ctl+A to Select All. Right-Click selection area - Actions > Convert To > JPEG.
I run Photoshop in Wine, so can verify that Kim will convert *.psd - to JPEG, TIFF, GIF, or PNG.
To resize at any of 6 Presets or Custom size - Actions > Kim Compress and Resize - and select Preset, or click Custom, and type desired size in the box.
WebExport sizes/conversions are at the bottom of that Submenu.
Those with P&S who end up after a day out with hundreds of JPEGs at, say, 3648 x 2736 - can copy their card(s) to a desktop directory, make a Copy of the Camera directory of JPEGs onto the desktop - and in that Copy directory (not your precious originals!) - Select-All - and convert to JPEGs at say, 1024 x 768.
That will make flicking through them in PhotoBook (etc) - much faster and easier. That can take several minutes for several hundred, so just do something on another desktop while waiting.
Regards, Dave.