adjust image layout

quad/qtest/test-jpeg.rkt

@@ -1,7 +1,11 @@
 #lang quadwriter/markdown
 
-Foo
+Here's a jpeg
 
 ![alt text](test.jpeg)
 
-Bar
+And a png
+
+![alt text](test.png)
+
+Done.

quad/quadwriter/layout.rkt

@@ -332,7 +332,7 @@
           (match-define (list line-width line-height) (quad-size line-q))
           (define new-size (let ()
                              (define line-heights
-                               (filter-map (λ (q) (quad-ref q :line-height (λ () (pt-y (size q))))) pcs))
+                               (filter-map (λ (q) (or (quad-ref q :line-height) (pt-y (size q)))) pcs))
                              (pt line-width (if (empty? line-heights) line-height (apply max line-heights)))))
           (list
            (struct-copy

quad/quadwriter/tags.rkt

@@ -32,9 +32,11 @@
 
 (define-tag-function (img attrs exprs)
   (qexpr (list->attrs
+          :line-height "false"
+          :line-align-last "center" ; need `last` because img is a block-level element.
           :image-data (second (assq 'src attrs))
           :image-alt (second (assq 'alt attrs))
-          :display (symbol->string (gensym))) exprs))
+          :display "block") exprs))
 
 (define-tag-function (br attrs exprs) line-break)