[In Omnibus]

You could use something like the following in your css_additonal.css template:

@media only screen and (max-width: 1080px)

.postbitlegacy .userinfo .postuseravatar img, .eventbit .userinfo .eventuseravatar img {
max-width: 75%;

@media only screen and (max-width: 768px)

.postbitlegacy .userinfo .postuseravatar img, .eventbit .userinfo .eventuseravatar img {
max-width: 50%;

@media only screen and (max-width: 480px)

.postbitlegacy .userinfo .postuseravatar img, .eventbit .userinfo .eventuseravatar img {
max-width: 25%;

You will need to adjust the percentages based upon what you want to see at each screen size.