I have to upload alot of product images to an eCommerce website. Dimensions are now 600x600px and image size 200-300kb. I want to batch reduce the file size of these to under 70kb.
Using Photoshop go to File > Scripts > Image Processor (this is essentially the same functionality as Bridge in Lucian's answer):
Here are a couple of simple JPGs – both 600x600, 258kb and 274kb respectively:
Here they are (after running them through the Image Processor) at 53kb and 74kb:
So this does just what you asked... but as you can see they don't look too good anymore... I had to lower the quality setting to just '1' to get these results. These are useless IMO. But this method would work for someone who didn't want to reduce their file sizes quite so drastically...
Running an action to 'Save for Web' in a batch (File > Automate > Batch) as described in this link, as provided by user244052 in the accepted answer above.
However when recording the action, in the 'Save for Web' dialog choose JPEG but then rather than using the presets and sliders as described at the above link – instead go into the little Optimize Menu flyout and choose 'Optimize to File Size':
And then enter 70kb, click 'OK' and then save... This is easier than trying to hit the sweet spot by trial and error. Also the Save for Web dialog includes the option to 'Convert to sRGB' so doing it as a separate step as described at the link is not necessary.
I would also examine all of the files you wish to process and the choose one that has a roughly average file size, then record your action on that image... This way your results should roughly average 70kb.
After optimization using 'Save for Web':
Those two weighed in at 59kb and 83kb respectively, and they don't look quite as bad
For those without Photoshop there is also this online tool: Bulk Web Resizer
Here's what it looks like:
And here are the results at the settings shown:
File sizes were reduced to 72kb and 86kb using this method.
Use Adobe Bridge
Drop all the images into one folder, select all of these in Bridge, right click and choose 'Export to → Hard Drive'. Then you get an option when you 'Constrain to Fit' all the images to 600px and try different values in the 'Image Quality' field until you manage to get these under 70kb.