This should work:
<%= render :partial => "info", :locals => { :img_style => "original" } %>
With this partial:
# infos/_info.html.erb
<%= image_tag(info.image.url, :class => img_style), info %>
However if you are calling the partial wrong then try this as the partial:
# infos/_info.html.erb
<%= image_tag(info.image.url(img_style)), info %>