Sorry, but I find your idea a) not really user friendly [actually rather user un-friendly] and b) additionally I would also consider your approach risky – very risky – even if you managed to solve the visibility issue.
a) While I suppose it is true that our visual memory works quite good – I fear it would be quite difficult to actually have enough unique/distinct images of fruits. A user might remember some 'round orange fruit' in his password – but was it an orange? Or was it that apple? Additionally images might slightly different under different conditions / on different devices. Screensize, brightness, contrast – all factors that will make this scenarion user unfriendly.
b) Even if you came up with 10 brilliant images of fruits – it's just 10 – which makes your passwords as safe as passwords that would only consist of digits from 0 to 9. That is a security risk – a really bad one. This is why many registration systems ask the user to use a combination of at least 6 numbers and letters – and often encourage people to also use some CAPS. This way a user can choose from at least 60 different "fruits". A six digit password would then have at least 46.656.000.000 possible combinations. The more the better.
c) Last but not least I would have a suggestion for the usability / visability issue: you could give the fruits letters (or numbers) and ask the user to enter those letter in the password field. But this would make your password protection as secure as a system where you could only use the numbers from 0 to 9. Seriously: only thinking of it gives me the creeps. I think I would probably not register with a service that would use such a system.
d) And another reason not to use a four digit fruit-string for a password: I suppose you don't want to have to look and feel of a slot machine?!