I use this add-on because it is about the only image resizer extension available for thunderbird. However, if often fails to offer to resize the images when importing an image. It also often pops up when we don't want to resize any images. I do not see why image resizing should be "automatic" (triggered when images are imported) instead of just adding an option when we click the image to resize it. Moreover, since it often fails, and because a user can change his mind, it should be possible to resize an image even after it is imported, which doesn't seem to be the case right now.
This review is for a previous version of the add-on (3.1.1-signed).