jensl

Allright, I changed it into
background: none;

That worked for me! Thanks for your support!

Thanks for your reply int3rc3ptor.
I’m also using the Mystile theme and found this code in my style sheet:

.post img,
.type-page img {
padding: .53em;
border: 1px solid #e8e4e3;
background: #fff;
max-width: 100%;
height: auto !important;
display: block;

Also removed the background color line but nothing did change.
Anyone knows how to fix this?

Thanks in advance.