I have been experimenting with using internal backlinks whenever a secondary keyword is used and link to another article within my site that is relevant to that keyword.

Example:

Main Keyword might be purple apples, secondary keyword might be purple apple pies. I would link purple apple pies to another article within my site about making pies

A good question which required a lot of discussion.but what i have learnt if you content is great and succeed to engage the visitors then it doesn't matter of images and links and you should have to provide the right and better navigational links which navigate visitor to best source of that topic for him. but if you talk about the low quality content then you probably add some creative graphical images to attract the visitors and on one page Google advice not to provide more than 100 navigational links...

I would suggest that you use internal linking as much as is needed. This is one of those factors that will benefit you more from making it user friendly, then it will from making it "SEO optimised".

This is all down to Googles latest updates revolving around rewarding sites that are quality and user friendly. You should try to make sure you are never 2 or 3 clicks away from any other page on your site (good side bars etc. help this).

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Those things never matters, unique content with quality backlinks are very much essential.

You have to need usefull article for those

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For internal pages Iwould recommend 5-6 links,and usually people click images/links as per their own choice.

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A page can have a total of 100 links or fewer per page which includes 80 internal links and 20 external links. Using your keywords as anchor text for internal linking will be more helpful. For further reference How many links per page?

Image and link both has the equal priority. I suggest you use both image and link for internal linking.

Ok, the answer to your question is.. 42! Just kidding.

Well, actually, as long as there is at least one internal link pointing to the required page , this already gives it a boost. And it's better if the link is a navigational one, but I would do it only when it looks natural.

See, when it comes to internal linking, it's more about whether the page is interlinked with the rest of the site or not, but not about the number of internal links pointing to the page.

As far as text link/vs image link is concerned... Well, for SEO purposes, a text link is the winner. But if you need to drive traffic from a particular page to another page, a image link could be more effective, provided it's clear that the image is clickable.

At-least one should be there so for the search engine crawling

Internal link is important for every website. Just make natural navigation for yours users, not only for search engine bots.

"How many links (internal page linking) do you recommend for an internal page??"

1 is an absolute minimum...
I`ve remember reading something about "100 maximum" in Google's guidelines, but I think you can go beyond that.
Keep in mind, SEO is about focus - I would suggest creating an internal hierarchy that will provide more links to your most important pages. Be smart about it. If you have 2 articles, one for 10,000 searches/month KW and the other for 500 searches/month KW, you shouldn't spent equal resources to promote both of them.

"And what people usually click? link or image?"
I wouldn't go as far as suggesting any "one-size-fits-all" rules here.
In e-commerce site, usually, an image will get more clicks but on Wiki site you`ll see more text-anchor clicks...
It depends, but having said that, you'll always need to consider such things as a focal point and call to action.
Explain to your visitor just what you want them to do and make it easy for them to do it. Keep it simple and interesting, and you`ll be rewarded.

I think it's better to link the text and picture. Have your bases covered!