If they aren't in a proprietary format (see Howitzer's comment about RAW files), then you might have a look at Irfanview.

It's not hard to set it up to perform a batch conversion which can include a number of steps, e.g., resize, change JPEG compression, rename files.

Once that were set up to your satisfaction, then you'd be able, when browsing a directory using Irfanview, to select a number of files, right-click, then select "Start batch dialog with selected files...". Yes, that launches a dialog, but it will start off (IIRC) with the settings last used - so you wouldn't necessarily be resetting everything every time you wished to do a batch job.