Set ALT attributes on img tags

Per 13.2 of the HTML 4.01 specification, the alt attribute on the
img tag is mandatory.

_index.html

@@ -63,7 +63,7 @@
 
 TITLE(Everything curl)
 <a href="https://www.gitbook.com/book/bagder/everything-curl/details">
-<img style="margin: 0px 0px 20px 20px;" align="right" width="200" height="262" border="0" src="pix/everything-curl.jpg"></a>
+<img style="margin: 0px 0px 20px 20px;" align="right" width="200" height="262" border="0" src="pix/everything-curl.jpg" alt="Everything curl book cover"></a>
 <p>
   This is a detailed and totally free book, available in ebook
   formats, <a href="https://www.gitbook.com/download/pdf/book/bagder/everything-curl">PDF</a>

_sponsors.html

@@ -19,18 +19,18 @@
 </div>
 
 <p>
-  <a href="https://www.haxx.se/"><img src="pix/haxxlogo.png" with="500" height="169" border="0"></a>
+  <a href="https://www.haxx.se/"><img src="pix/haxxlogo.png" width="500" height="169" border="0" alt="HAXX logo"></a>
 <p>
  Haxx is the primary sponsor of the curl project.
 
 <p>
- <a href="https://www.mozilla.com/"><img src="pix/mozilla.png" width="500" height="143"></a>
+ <a href="https://www.mozilla.com/"><img src="pix/mozilla.png" width="500" height="143" alt="Mozilla logo"></a>
 <p>
  Mozilla employs <a href="https://daniel.haxx.se/">Daniel</a> and lets him
  spend parts of his paid work hours on curl.
 
 <p>
- <a href="https://www.fastly.com/"><img src="pix/fastly.png" width="500" height="263"></a>
+ <a href="https://www.fastly.com/"><img src="pix/fastly.png" width="500" height="263" alt="Fastly logo"></a>
 <p>
  Fastly delivers the curl web site and web contents to the world.