Attachments should open using whatever you set Thundrebird to use. Settings > General scroll down to 'Files & Attachments' under 'Content TYpe' locate the JPeg option JPEG image on the right under Action click on the drop down chevron and select the desired option eg: Use Photos (default) or another option. If Jpeg image is not in the List and you have scrolled up and down to find it - it was at the bottom of my list, hidden until I scrolled down - then you need to set up. In email right click on 'Attachment and select 'Open' It should ask open with what - you need to choose the appropriate program Also select checkbox 'Do this automatically for files like this from now on' click on 'Ok'