Check out soared's comment above. Basically, 1-pixel images like these are used by companies to track when someone views their content. It's a very common practice - most of your emails probably contain tracking pixels so that whoever sent them can tell if you viewed the email. If they see a request on their server for the URL they put in the tracking pixel, they know you've opened the email.

In Bylock's case, it was most likely a tracking pixel embedded in advertisements for Bylock that the other apps were displaying. The pixel helped Bylock track how many people were viewing their ads.