Attached pictures not shown at recipient

Solution 1:

If those images are taken with an Apple device, since iOS11 those devices have defaulted to HEIC format for photos. However, this format (and codec) are not visible on Windows by default to date. Depending on how you attach those images in macOS, you may be sending the HEIC version instead of JPG. Since macOS doesn't need to use .jpg or .heif to identify formats, it is quite easy to send images not realizing they are HEIC.

To avoid sending HEIC, (especially if you are having issues), you can change iOS camera to capture in JPG instead, using the 'more compatible' setting in Camera settings.