Doing a grep for `alt=` in the codebase it looks like we're doing this in a few places:
```
cms/templates/edit-tabs.html:86:    <img src="${static.url("img/preview-lms-staticpages.png")}" alt="${_('Preview of Static Pages in your course')}" />
cms/templates/howitworks.html:33:            <img src="${static.url("img/thumb-hiw-feature1.png")}" alt="${_('Studio Helps You Keep Your Courses Organized')}"
 />
cms/templates/howitworks.html:67:          <img src="${static.url("img/thumb-hiw-feature2.png")}" alt="${_('Learning is More than Just Lectures')}" />
cms/templates/howitworks.html:101:            <img src="${static.url("img/thumb-hiw-feature3.png")}" alt="${_('Studio Gives You Simple, Fast, and Incremental 
Publishing. With Friends.')}" />
```

so I thought this was fine. Can you do a bit of digging (maybe talk with Ned for advice) and figure out what's the right thing to do? May involve (separately) updating the i18n guide to reflect what does & doesn't work.