I myself get confused with colours such as green and red and sometimes have difficulty reading these images. It is also not clear how i should type them as some sites do not say if it is case sensitive. This is all very picky and doesnt affect the opinion of the site but i really do not like having to guess at letters and characters that i usually get wrong. When i do get them wrong the page usually refreshes clearing certain fields on the registration form. If this happens more than 3 times i usually give up with the site.
However there is one thing i really disagree with and that is this:

I apologise for the quality of the picture but you can see the box in the middle. Suddenly im having to do maths to get into a site which is rediculous. whats more is that i am unsure how to type the answer in. Do i type it in words, or perhaps numbers themselves.
Soloution
I do have a soloution however and that is this:

What is this?
answer: apple
a robot would have great difficulty deciding what this is but we can do it in an instant. This however is still confusing becuase people may type in hand or use capitals so apropriate images must be used and the case in which to enter them clearly shown