Doing PhD on computer vision with an engineering background
Firstly, I would definitely recommend taking an online course (a commenter mentioned one from Coursera, and EdX also has some good ones as well) associated with a reputable university. Taking some kind of structured course designed to build a fundamental understanding of the topics and math involved is a great way to jump in and get started.

Once you do that, you'll know where the biggest gaps in your preexisting knowledge are, and you can address those with books/resources covering the theory and math involved, and other resources involving the practical application of what you want to do.

Since you mentioned in interest in pedestrian tracking, a topical and interesting thesis question could concern pedestrian tracking by an autonomous car, and an algorithm for reading "clues" from pedestrian motion to detect what they are likely to do next so that the car can make a choice about whether to slow, stop, or keep going and monitoring the situation. (Just an idea, but there's tons of practical applications for being able to track things).

Once you gather some fundamental information, and know what direction you want to go in, you would probably be in a position to start talking to some professors. If you can demonstrate that you understand the concepts involved, that you have an idea about what you want to research, and that you're serious about doing what it takes to be successful, most professors would be willing to listen and advise you from that point forward. The idea you bring to the table at this point does not have to be your final research idea, but bringing something realistic that you're interested in allows you to both show that you're serious about the research, and also to have a starting point for coming up with a final research plan with input from an experienced researcher.