Hi there,
So you do right by practicing with the how2become stuff, it is really good.
If there's one part you're struggling with, all I can suggest is to keep practicing. These tests are aptitude tests, not skills tests - so unfortunately, if you're not naturally apt in a certain area, they are designed to filter candidates out. Other than practicing, its very hard to suggest any other strategies.
It was a very long while ago now that I did my drivers aptitude test so I'm probably not the best person to answer about current tests, though I must say I can't actually remember what the images were that flashed up (that bodes well, doesn't it?!).
All the best with the assessments, it sounds as though you're doing absolutely the right thing by taking them seriously and investing in the preparation materials.
Finally, this thread will likely soon be moved into the careers section of the website, if you're struggling to find it.