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Attachments have been used on the navigation list for quite a few years to
good effect, but anything over a megabyte in size can cause problems. I
strongly recommend limiting any attachments to 250k or smaller. Truth is,
almost all of the large files that have been sent to the list in the past
few years could have been reduced in size by a factor of 10 or even 20.
Here's some practical tips:
1) Is it an image? Probably it is and these instructions will assume it is.
An exception would be something like a complex document, perhaps a chapter
from a book with mixed text and graphics. You will need some basic image
manipulation software to modify an image file in order to ensure that the
file size is reasonable. Most scanners and many digital cameras include
basic software for this purpose.
2) If you have scanned or photographed something it should start out as
24-bit graphic file (a "true color" image). If you got the image from some
other source (be careful regarding copyright), it's possible that you might
have to increase the color depth to 24-bit. At this full color depth, you
should reduce the size of the image until it just barely makes the point
you're trying to make (text should be readable if it's a scan of a table for
example, but we don't need to see the small details of the font used to
print the original). Most image manipulation software will let you change
image size by some percentage. Try reducing the size by 25%. If you can
still read it, repeat the reduction. You can get big savings in file size
here. If an image is reduced from 1200 pixels by 800 pixels to 300 by 200,
the resulting file will typically be 16x smaller (a factor of four in each
dimension).
3) The next step is to reduce the color depth (but see below). You don't
usually need 24-bit color in the final image. Try reducing the depth to
8-bit or 4-bit (corresponding to 256-color and 16-color images
respectively). Again, this is a standard feature of most image manipulation
software. Note that the size reduction for the final file can be significant
here. The math is straight-forward in simple cases: a 24-bit image uses six
times as much space to store each pixel in the image as a 4-bit image. You
could go further and reduce the color-depth to 2-bit (black and white
usually) for a high-contrast diagram but this frequently makes scanned
images harder to read.
4) Decide how to save the image. If it's a photograph of a ship or a scene
or even a sextant, you should probably save the file as a JPEG/JPG. Note
that you don't have to worry about color depth for this file type. If it's a
scan of a page of text or numbers or a diagram, you should probably save it
as a GIF image. Yes, there are other image file formats that have their
uses, but for the typical uses we see on NavList, JPG and GIF will always
work just fine.
5) Avoid "file wrappers". A number of recent attachments have been saved as
PDF files or DOC files. These do almost nothing except add to the file size.
Some scanner software will recommend a PDF as a preferred format. For
multi-page scans, this file format adds a little convenience but the cost in
file size can be great.
6) Finally, bear in mind that email attachments are converted, usually using
"Base64" encoding when they are sent. Every three bytes of binary data is
turned into four bytes of text (this is usually hidden from the end user by
the mail reading software). So a 300k image attachment is really 400k of
email data.

You might wonder why we should worry about file sizes of one megabyte and
smaller when millions of people are viewing video files on YouTube every day
that are hundreds of megabytes in size. I would say that it's because the
Internet's email system is relatively old-fashioned and simultaneously
critically strained. There is considerable range in what is considered
acceptable email use on different systems. Some email services will reject
out-right anything larger than a megabyte or two. Others will charge their
users if they exceed a rather low limit. And some users simply have no
convenient means for skipping over large downloads that don't interest them.

I do support the use of attachments for appropriate content. Reducing file
size can go a long way to answering the above concerns.

 -FER

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