When you do something like <a href="http://samplewebsite.com/homepage.html"><img src="/sample.gif"></a> Code (markup): And it is put on someone else's website, does the link get credit when counting up your pr?
yes, but you should also have alt tag to give. Backlinks are always counted (effect can be vary according to the quality of webpage you are linked, positiong etc) Alt tag would help your link to be indexed in search engines for the keyword(alt text) you are looking to promote for your website.
Getting image links will definitely helpful, it will create diversification in your links and this very good from SE point of view.
I guess it does, otherwise, why would all those widget services embed their links in the to-be-pasted 'add this to your blog' code?