There was 
an issue opened at github, asking how to display images without any margins.
I think this is more a usage case than a bug in AnkiDroid, so i reply here.
Doing this is actually a bit tricky, as there are a few places where this margin can be set, and you have to 
UTSL to find some of them.
And i needed 
SuperStyler add-on to work out the details.
In the end i came up with this CSS:
.card {
    font-family: 'Linux Biolinum O';
    font-size: 24px;
    text-align: center;
    color: #657b83;
    background-color: #fdf6e3;
    margin: 1ex 0.5em;
 }
.card img {max-width: 100%; max-height: none;}
.mobile .card img  {max-width: 100%; max-height: none;}
.mobile .card {margin: 0.5ex 0em;}
.mobile #content {margin: 0.5ex 0em;}
It looks like you have to set the margin twice for AnkiDroid for it to work. Of course you can use just {margin: 0;}, but i still like a little vertical margin. Again, you can play around with the CSS using SuperStyle to find a style that matches your taste. (You may have to open a port in the firewall.)
The images are with and without the CSS tweaks.

