Can't see images in mails I compose
To make a signature with images go to Preferences under the Mail menu and click on signatures. You should get a panel that looks like the image below. In the area on the right, right-click and select Import Image. You can select the image you want in your signature.

You can also open an image you want to display. Select all and copy the image. You can then go to the panel shown below and paste into the area on the right.

To solve your problem about images not appearing you can try, instead of loading image into your message, the same method as above. Open the image and copy it, then paste it into your email.