Add alt-text for more images

[misaugstad]

Brief description of problem/feature

I've been trying to add alt-text to images as I notice them (which improves the functionality of screen readers on our site). But there are still plenty without alt-text (or it has alt-text but no Spanish translation). We should do a full inventory and fix such cases.

[misaugstad]

Well I think that doing this for the landing page is important at least! Although the actual data collection is a visual search task and therefore has less need for being screen reader friendly, the landing page tells you what Project Sidewalk is all about, so I think it makes a ton of sense for it to be very screen reader friendly.