When you are writing the message on eBay, there is an "Attach photos" button next to the send button. If you click that, it will open a dialogue box - click Ok (after reading the links to requirements etc), then either drag and drop images into the field, or click 'Select photos from your computer' to open the file explorer and double-click the images you want to upload from there.
If a member has selected to receive emails via eBay as text only (no HTML) you won't be able to send them a message with images.
