Icons in legend missing alt text

The legend on the Elysia Cron configuration page is missing alternative text on the icons. This could prevent an administrator who uses a screen reader from being able to match up the icons on the various cron tasks with the explanations of the icons.

Steps:
1. Go to /admin/config/system/cron on a site configured with Elysia Cron 7.x-2.3 and many cron jobs
2. Examine source code
3. Find the source for the legend

Actual results:

 <thead><tr><th>Legend</th> </tr></thead>
<tbody>
 <tr class="odd"><td><img src="https://sfmtaweb.prod.acquia-sites.com/sites/all/modules/elysia_cron/images/icon_idle.png" alt="" /> - Job shouldn't do anything right now</td> </tr>
 <tr class="even"><td><img src="https://sfmtaweb.prod.acquia-sites.com/sites/all/modules/elysia_cron/images/icon_waiting.png" alt="" /> - Job is ready to be executed, and is waiting for system cron call</td> </tr>
 <tr class="odd"><td><img src="https://sfmtaweb.prod.acquia-sites.com/sites/all/modules/elysia_cron/images/icon_running.png" alt="" /> - Job is running right now</td> </tr>
 <tr class="even"><td><img src="https://sfmtaweb.prod.acquia-sites.com/sites/all/modules/elysia_cron/images/icon_disabled.png" alt="" /> - Job is disabled by settings, and won't run until enabled again</td> </tr>
 <tr class="odd"><td>Notes: job times don't include shutdown times (only shown on channel times).</td> </tr>
 <tr class="even"><td>If an abort occurs usually the job is not properly terminated, and so job timings can be inaccurate or wrong.</td> </tr>
</tbody>

Expected results:

<thead><tr><th>Legend</th> </tr></thead>
<tbody>
 <tr class="odd"><td><img src="https://sfmtaweb.prod.acquia-sites.com/sites/all/modules/elysia_cron/images/icon_idle.png" alt="Idle" /> - Job shouldn't do anything right now</td> </tr>
 <tr class="even"><td><img src="https://sfmtaweb.prod.acquia-sites.com/sites/all/modules/elysia_cron/images/icon_waiting.png" alt="Waiting" /> - Job is ready to be executed, and is waiting for system cron call</td> </tr>
 <tr class="odd"><td><img src="https://sfmtaweb.prod.acquia-sites.com/sites/all/modules/elysia_cron/images/icon_running.png" alt="Running" /> - Job is running right now</td> </tr>
 <tr class="even"><td><img src="https://sfmtaweb.prod.acquia-sites.com/sites/all/modules/elysia_cron/images/icon_disabled.png" alt="Disabled" /> - Job is disabled by settings, and won't run until enabled again</td> </tr>
 <tr class="odd"><td>Notes: job times don't include shutdown times (only shown on channel times).</td> </tr>
 <tr class="even"><td>If an abort occurs usually the job is not properly terminated, and so job timings can be inaccurate or wrong.</td> </tr>
</tbody>

Note: The alternate text, e.g., alt="Idle", already occurs on the rows for the cron jobs themselves.