If your .jpg files have a custom preview icon you can use that to replace the generic folder icon.
Open the Get Info window for each.
In the upper left corner is a small icon preview for each. Click the image and a blue box will now border it.
Command-c to copy. Switch to the Get Info window of the folder icon and single click to highlight the icon.
Command-v to paste.
I have set all my jpeg files to 'show icon preview'.
You mention jpeg files having a '*custom preview icon*', but whenever i select '*get info*' for a jpeg, the image in the top-left corner is always the standard jpeg logo and does not show the actual image that the jpeg is.
I now understand (thanks to your previous reply) how to replace one image with another, but first the top-left corner of the '*get info*' window of the jpeg needs to show the actual image.
Custom icons are stored in the "resource fork" on the Mac OS. If your images do not have one you'll need to use an image editor like GraphicConverter ($35 shareware with a free demo) to create one.
http://www.lemkesoft.com/
it doesn't quite work the way you said it should though.
You have to actually open up the jpeg with a dbl click, '
THEN' do apple-C to copy the jpeg image.
Then select the '*get info*' option for the folder you wish to copy the jpeg image onto.
When you see the blue folder image in the top left corner of the 'get info' window of the folder, click on the folder to make it highlight blue, then select apple-v to paste the jpeg image onto the folder.
That's how a friend told me to do it, and it works fine.
There is no need to open the 'get info' window of the jpeg, you only need to do that for the folder.