Wally wrote:
I have a couple hundred JPG files of different aspect ratios. Hardly
any two are alike in dimension. Mostly they are approx 4:5, but that
is rough. They were just cropped over the years for best display on
the screen.

Now I want to make prints of them, and the prints need to be 4x6 inch
because that is the standard and cheapest, and will fit into the most
common albums.

You could consider making an album not by sticking photos in
it, but by having the *pages* (containing the images) printed
(or even better, chemically developed like real photos) and
then bound to a book --- presto, an album and you can keep
all the irregular dimensions.

Instead I would like the whole file printed, leaving white margins (or
some other color) on either side. But those places won't allow me to
do that.

imagemagick. It's made for batch processing of images of all
kinds. (And it's open source, so you can download it, use it
and even improve it as you like.)

You'll need to read the manual to find the right options and
sequence of commands, but someone will have had a similar
problem before, so it's likely you can find an example on the
web that you can adapt.

One way would be to resize (keeping aspect ratio) the image
to 1200x1800 max (300dpi for 4x6 inch), then extend to
1200x1800 around the image.

(untested
convert original.jpg -resize 1200x1800 -gravity center -background darkgrey -extend 1200x1800 resized.jpg

Now make a loop over all your jpegs, creating the resized jpeg
in another directory ...

-Wolfgang