Where did you get that info from? Please provide the source. 

lol all images should have alt values 
but u can use any online html validator and you will get the same error
a good one i like use is powermapper 
http://try.powermapper.com/Demo/SortSite

put oxwall demo site into it
http://demo.oxwall.org/

and test it 
a good site will score at 30% errors and sad oxwall is 90%
at 30% or lower google loves you 

i proved it to a costumer long time ago has website page was rake horable in google 
it tested at 80% 

i then made another page exactly the same but fixed all the error and got it to 28% error
and it shot to the top google in 3 weeks 

googles pet hate have alt tags for screen readers for blind ppl 

No,  you're talking about the validation now that's a different matter, your original post says each a element must contain text or an IMG where you got the information from?  please provide the source

i did provide the sorce
http://try.powermapper.com/Demo/SortSite

and its right 
class="ow_logo_img"> is the logo image Each A element must contain text or an IMG with an ALT attribute.
says it should contain text or a image with alt now its not a text link its a image link so image should have alt tags

if you get the script down to 30% errors you get a badge that you can put on the front on the oxwall site here

would tripple the number of ppl use oxwall  

so it should be like this
<a href="{$siteurl}"" ><img alt="{$sitename}" width="40" height="40" src="{$ow_logo_img}"></a>

and yes all images should have width="" height="" values