<%= image_tag(url_to_avatar(@user), :width => 240, :height => 240, :alt => "头像") %>
尺寸：240*240像素
<%= image_tag(url_to_avatar(@user), :width => 100, :height => 100, :alt => "头像") %>
尺寸：100*100像素
<%= image_tag(url_to_avatar(@user), :width => 40, :height => 40, :alt => "头像") %>
尺寸：40*40像素