Acymailing is working correctly when you can see the images in Gmail and Yahoo.
The question remains why abv.bg (I guess this is a webmail client?) is not showing the images. That could be a setting or limitation in abv.bg or your server is blocking the image requests abv.bg sends to your website. In that case you could look in your console if you see any error messages when opening your mail. Otherwise you could inspect your server log how it handles requests from abv.bg.
You could also try to use another browser to read your mail at abv.bg.