Here are a couple of things that might help where I believe the accompanying solutions have helped some of my PC friends who have had problems to visualize or open or download my attachments:
1. Issue #1 Sometimes I believe the recipients might have difficulty with an attachment if one is:
a. copying and pasting or even dragging attachments from a previous E-mail to a new one that is being composed. I think that there is also sometimes a problem if one tries to copy an image from a letter in the Mail program's viewing pane rather than first opening up the original E-mail to its own window.
b. Trying to copy a picture of an opened jpg file (while it is viewed in the Preview program) and then trying to paste this into the E-mail. Depending on the settings, you then could for instance be inserting the image now as a TIFF rather than as a JPEG into the new E-mail without realizing the change. This has caused some of my PC friends troubles.
Suggestions to resolve issue #1:
a. Open up the original E-mail in its own window and then copy and paste or drag the attachment from the first E-mail to the new one. Do not copy and paste the image of file to the new E-mail.
b. Do not open Preview and then copy and paste the image. Instead drag the icon of the .jpg file from the desktop or wherever you have it stored into the new E-mail.
2. Issue #2: I have had problems for some recipients of files who have PCs for opening files if the extensions of the files are not visible, because some PCs require a visible file extension in order to work correctly. Thus before attaching the jpeg files to your E-mails, even if I have been able to see the .jpg file extension in a file's name, the extension may not really remain visible once the recipient who has a PC actually gets the E-mail. I think this can occur if the Macintosh that is sending the E-mail has the general Finder Preferences set to "Show all filename extensions" but the .jpg files themselves have been created with the setting for each individual file so as the extension is hidden. (Remember: You are able to see the extension in the file name, solely because of the settings for the finder, and thus you may not realize the extension is set for the .jpg file to be hidden).
Suggestion for solving Issue #2: Thus while you look at the files' icons on the desktop, even if you are able to see the .jpg extension in the file name, go ahead and make sure the .jpg files are really set to show the extension. To do this, highlight the icon of the .jpg file, and select "Get info" from the "File" pull-down menu (command key and I key), and while the carrot for "Name and Extension" is turned downwards (click on the carrot to change its orientation) then be sure the checkbox for "Unhide Extension" indeed is unchecked. With this unchecked, then finally attach the file either by dragging or copying and pasting the icon into the E-mail (I believe you should not try to copy and paste the picture from the window of the opened jpg).