No alt text for images on events page

[jchristgit]

As pointed out in #1363:

Many images on the events page lack a definition for alt text.

All images need alt text defined so that screen readers can present the information to the user. If an image is not necessary for a screen reader user, it should define alt="".

[DragonSenseiGuy]

@jb3 I'll take a look at this