try Irfanview, it's free and does the job. I found the batch conversion very convenient, just select all pics in a folder and set the options of which there are quite a few and it will resize, rename etc and drop the new images into another folder. I take it most other editors have this facility, I just found this handy when resizing hundreds of pics. Very easy to use.
I echo the advice to use Irfan View. Batch operations won't get the best results for all images, though its fine for the average photos.
The degree of compression you can use depends on the image data. For example, most photographs still look good when compressed quite a lot - say to a setting as low as 35 in Irfan View (though I would recommend 50 for general us, and 75 for high quality). Screen grabs, or images with sharply contrasting areas of colour, don't compress very well at all. It is often better to use GIF or PNG instead for such images, both for smaller file size and lossless image quality.
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